In animated works, pictures by definition have to move, which means that the more detailed a given frame is, the more time and effort it takes to produce, and the more visible any mistakes made in color shading or lighting will be. As a result, even in the most well-crafted animated works, frames of actual animation footage may intentionally leave out subtle details to make the end result animate more smoothly.
On the other hand, covers and other static art don't have that problem. They are just one picture, so the production company can afford to invest more effort in them. Take the page picture, for example: The top part is a frame taken during the actual movie, and the bottom is the exact same scene redrawn for its inclusion in The Merch. Colors are brightened, textures are defined, surfaces are given depth, and extras are drawn in everywhere. Bonus points if the box cover is sparkly or "metallic" in some way so that it shimmers in the light.
Disney defined this style for their home video releases of the Disney Animated Canon to such an extent that similarly detailed covers for other traditionally animated movies not from Disney end up falling into the All Animation Is Disney trap as a result.
This can also apply to comics when comparing the interior pages to the cover: The interior pages require the artist(s) to distribute their efforts across multiple panels, whereas the cover art is created separately, so the cover artist(s) can invest a greater amount of effort and detail.
For manga, the poster-like artwork between the chapters proper are called Splash Pages◊, whereas the anime equivalent (just before and just after commercial breaks) are called Eye Catches, respectively. Bonus Material art is often more detailed as well. Most Mecha have 'mechanical designs' and 'animation models' where the latter is a simplified-for-animation version of the former.
This also applies to a lot of video game box art and advertising from the 1980s, especially before it became possible to capture high-quality gameplay images off a TV.
Remember Tropes Are Not Bad. This is simply a result of limitations with even the best animation.
Contrast Minimalistic Cover Art.
- The cover of anime film Space Thunder Kids DVD contains rather high quality drawings of things that have nothing to do with the very sub-standard animation.
- Many comic book covers from the 1980s onward are basically ten years ahead of the interior in terms of color complexity.
- The covers of the Buffy the Vampire Slayer Season 8 comics are near-photorealistic renditions of the actors, while the actual comics are much less detailed and much more stylized. Though there are also variant covers drawn by the interior artist, which give a more realistic image of what's inside.
- The covers of newer Monica's Gang comics are more beautiful and well-detailed in terms of tracing and shading than anything you will find inside it.
- Basically all Disney works have this, but it especially applies with their low budget Direct-to-Video films.
- Happily Never After and Space Chimps have covers that are designed exactly as the films are. However, the sequels' covers have the exact same design style as the first film while the animation of the film itself is on par with Vídeo Brinquedo.
- Alice in Wonderland did this with the opening credits, showing art that showed a lot of detail not apparent in the film proper. One included the overskirt on the queen's dress being trimmed with ermine (which is reflected in some of the merchandise and mascot costumes), when it looks like white piping in the movie.
- In general, a lot of old video game covers using original artwork succumb to this, as the games were incapable of such visuals. It wouldn't be until the seventh generation of consoles that they started truly looking like the boxart if they were using a realistic art style.
- The Mega Man box art for the NES and the cover of the PC version of The Secret of Monkey Island, both of which feature far more advanced graphics than the actual games do. Interestingly as both series developed and graphics improved, Mega Man was made to look more like the in game graphics and the Monkey Island characters were made to look more like the cover art.