What's the out-of-universe reason for the group's creation? There are at least three different explanations out there – note that they don't contradict each other, so they might all be true for all we know:
The arguably most cited reason is that company owner/publisher Martin Goodman wanted a clone of Justice League of America, one of the Distinguished Competition's biggest hits. Well, another Expy, given that Fantastic Four had already come into being that way.
Another explanation is that Marvel's fans loved Crossovers, so Stan Lee was inspired to do a permanent one.
Finally, it has been said that Daredevil was meant to launch in The Avengers's place, but got delayed due to Bill Everett's personal issues. Because print time was very valuable back in the day and couldn't easily be canceled, a replacement was quickly created – note that the first issue is exclusively about characters who already existed.
Prior to 2015, when the Fantastic Four comic was cancelled and he joined the Uncanny Avengers, the Human Torch was the only standard Fantastic Four member to not join the Avengers, and was in fact the only Stan Lee created character to have never done so. There have been many stand-in members of the Fantastic Four over the years that have also been Avengers: She-Hulk, The Incredible Hulk, Spider-Man, Wolverine, Luke Cage, Crystal of The Inhumans, Namor, and Ant-Man II.
The original, World War II-era Human Torch was also a member of the Avengers even though he isn't as famous.
In the Fantastic Four The End mini-series, Johnny is the leader of the Avengers in the future.
Approval of God: It was Roy Thomas who first had the idea of the Vision being the Human Torch in a redesigned body, but never got the chance to work on that story. It was Steve Englehart who finally wrote it down, in The Celestial Madonna Saga, with Thomas' blessings.
Author's Saving Throw: Issue #200 became infamous for Carol Danvers suddenly becoming pregnant, giving birth to a boy who grows to adulthood within a day. The Avengers' reaction is to throw her a baby shower. The boy, identifying himself as Marcus, explains how he Mind Controlled her and impregnated her with himself as a method of crossing dimensions. At the end of the issue, he returns home and Carol decides to follow him. Her teammates see nothing wrong with this. The story was widely decried by fans and critics and became known as "The Rape of Ms. Marvel". The writers and editorial staff behind the story later officially apologized for it. Chris Claremont subsequently wrote Carol's return, where she gives a What the Hell, Hero? speech to the Avengers for letting her leave with Marcus when he was controlling her mind.
Monica Rambeau might just be his favorite among Roger Stern's Marvel creations, and is unquestionably his favorite among The Avengers. He spent dozens of issues developing her from a frantic confused rookie into a leader with the veteran Avengers' full confidence.
Al Ewing loves writing Monica as much as Roger Stern and adds her to any team he writes if it is at all possible, including Mighty Avengers.
Roy Thomas didn't care for Quicksilver, which is why he wrote the character out of the book twice. Thomas apparently wasn't the only one who felt this way, as there was an internal company memo from 1972 that ranked the various Marvel heroes in order of importance, with Quicksilver coming in dead last among the Avengers.
Editor Mark Gruenwald disliked Monica Rambeau and, once he decided to start making her look bad, the character never managed to recover during her Avengers run. She continued to suffer for two decades after it, only starting to come back into prominence with Nextwave three decades later. First Gruenwald drafted a story that showed her to be an incompetent leader, in direct contrast to the two previous times she successfully lead the team, and fired Roger Stern for protesting it. She was so ineffective The Avengers temporarily disbanded. Then he had her depowered in a losing fight that left her emaciated and bedridden. Then she was kept from returning from the team by her mother, despite being an adult who was in freighting and law enforcement before her superhero career. Then she lost the title of Captain Marvel to new character Genis-Vell, then she lost her second superhero name Photon to Genis! All because Gruenwald was also the writer for Captain America and didn't like it when Avengers writer Roger Stern had Captain America take orders from a novice superhero.
Old Shame: "The Avengers #200" is the single issue of the original comic line that ran from 1963 and into the 1980s that Marvel most wants to forget. It was so unpopular even when it came out in 1980 that Marvel immediately scrambled to address it in "The Avengers Annual #10" a year later.
Screwed by the Network: The first issues (of the Lee-Kirby era), translated in Spain. For some reason, Avengers #9 was not published, they moved from #8 to #10. Initially, nobody noticed anything wrong (at that point, Status Quo Is God), but in later issues fans began to notice recurring flashbacks about the death of a "Wonder Man", which was not in any of the issues they had.
Shrug of God: When asked how The Avengers #200 even came to be, Jim Shooter claimed he didn't entirely remember how it got published or who came up with the original idea. He nonetheless decided to take some of the heat, acknowledging that since he was the editor at the time, and his name was in the credits, he obviously wasn't innocent himself, no matter if the idea came from him or not.
Technology Marches On: In the first Avengers story, Rick Jones tries to send a radio message to the Fantastic Four about Hulk, but Loki prevents it with his magic, and redirect it to Thor's radio instead. And, by accident, it is also redirected to the radios of Iron Man, Ant Man and the Wasp. The crossover Avengers/Ultraforce, made in the 90s, included a new universe with a revised origin for the Avengers. The radio message was replaced with an e-mail (and yes, it also reached other people as well, somehow).
Why Fandom Can't Have Nice Things: When Jim Shooter wrote Avengers #213, where Pym attacked Jan (something that he Never Live It Down), he received loads of mail complaining about it. He turned to Stan Lee for advice. Lee told him that he got a similar backlash about Spider-Man's love relations back in the day, and asked about the sales. They were growing. Lee told him that means that he should ignore the mail; if sales were growing it can only be because people like the stories, despite the casual complainers.
Writer Conflicts with Canon: In Avengers #213 Henry Pym gave a right-cross to the Wasp. The scene became one of the biggest examples of Never Live It Down in comics, and forever influenced reader perception of Henry Pym. Writer Jim Shooter claimed years later that it was all a misunderstanding. He said "there is a scene in which Hank is supposed to have accidentally struck Jan while throwing his hands up in despair and frustration—making a sort of “get away from me” gesture while not looking at her. Bob Hall, who had been taught by John Buscema to always go for the most extreme action, turned that into a right cross! There was no time to have it redrawn, which, to this day has caused the tragic story of Hank Pym to be known as the “wife-beater” story". His explanation, however, does not make sense with other scenes. Before it, there is build-up to their relation being in crisis. After it, Jan shows up in the Avengers mansion, revealing a badly bruised face, to the horror of Thor ("Odds blood! her face! What's the meaning of this? Did... he strike thee, woman?"). In the last scene, the Wasp refuses to cry over what happened. In the following issues, the Wasp ends their relation and starts divorce proceedings. In issue 224 (still written by Shooter), Scott Lang sneaks into prison to hear Hank's version of the story, and Pym says "The tabloids got most of it right, Scott! I was an idiot! I let my personal problems overwhelm me! I got in trouble and took it out on... on Jan!". The arc makes perfect sense with the scene being what we saw, but makes little or no sense if the scene was what Shooter claimed it to be. Add to that that the artist Bob Hall pointed that Shooter never voiced him any complain about the scene.
Writer Revolt: After Mark Millar big success with Ultimate X Men, Marvel proposed that he write a spin-off comic, Ultimate Wolverine, but Millar wanted to use the Ultimate The Avengers instead. Kurt Busiek, the writer of the Avengers at the time, did not want that to happen, as he feared that the regular Avengers would be left in the shadow of this new comic book. As the Ultimate Marvel universe was turning into a Cash-Cow Franchise, so badly needed by Marvel to get rid of the risk of bankruptcy, they allowed Millar to work with the Avengers. The new team got a different name, as Busiek requested, and was named "The Ultimates". This was not enough for Busiek, who resigned from writing the Avengers as a result.
.