Pennywise is a non-material entity from beyond the edges of the universe, merely projecting a body into this world. In what sense is a giant spider appropriate as its 'true' form? Because it's a creature that feeds on and draws strength from fear. What's the single most feared and reviled creature on Earth?
The book makes it pretty clear that IT's true form is not really a spider, that's just the closest approximation their human minds could come to. Though you're probably right about the "why", spiders are terrifying.
Last time I checked, spiders were helpful pest controllers and not evil eldritch abominations. Poor arthropods...
That doesn't change the fact that a huge percentage of the human population are terrified of them.
Which is probably the reason It's closest approximation is a spider - it's probably one of the most common things for humans to have a fear of, legitimately or not.
Why does it matter (in the book, anyway) that none of the Losers have kids? Because having children means having to put aside your own interests to focus on your children. Bill, Beverly, and Eddie are emotionally locked at the ages they were in 1958, Bill because he's still grieving for Georgie and hurting over his parents' distance from him, Beverly because her abusive husband regresses her to the age she was when her father was abusing her, and Eddie because his overbearing wife is a carbon copy of his overbearing mother. Ben, Mike, and Richie have made a living out of their childhood pastimes instead of outgrowing them. And Stanley, who did grow up, couldn't handle the impossible reality of what he'd gone through, and killed himself. This even applies to Henry, who didn't have the opportunity to have children, and stayed emotionally stuck as a twelve-year old; he's able to be empowered by IT because of his childlike ability to understand that it's not really his dead friends he's talking to...
Hell, this may even apply to IT. The Losers are only able to beat her as completely as possible after she's laid her eggs; her mentality is very much that of a cosmically spoiled brat for most of her appearances. But at the end, she rushes back to directly engage the Losers, even knowing how they can beat her, because she can feel Ben crushing her spawn.
Most likely the reason why none of the Losers can have kids is that if they did having the responsibility of having a kid to raise would keep them from running off to Derry on a potential suicide mission to take out IT, and if even one of the Losers with the exception of Stan didn't go to kill it, the very specific set of circumstances that need to occur for IT to be killed once and for all couldn't happen.
The full dramatic irony of the Losers' club: they really are losers. They lost each other. But they lost each other because they won.
IT is the most Dark Tower of the non-Dark Tower books. The Losers' club is a Ka-Tet and their cyclic battle with IT is a prime example of how Ka * the DT universe's form of fate works; the same events have a tendency to recur during both their encounters in 1958 and 1985, they can feel the force of the White when they form the circle, they can share Khef when they are together, and towards the end they feel Ka-Shume. IT itself is a creature from Todash space with its true form, the Deadlights, circling the outside of the Tower.
IT could be viewed an Evil Counterpart to the Christian Trinity. There's the "Father" (IT's Giant Spider form and its ultimate physical manifestation), the "Son" (Pennywise, who is IT in "human" form), and the (in this case, unholy) Spirit (the deadlights, IT's non-tangible form).
Or that It represents the devil, the "Other," God, and the Turtle, Jesus.
So, why now, 1957—1985? It has been around for millions of years. Why has this mysterious "Other" drawn the Losers together to finally fight It and take It down? Because, at this time and no other, It can't simply pack up and leave Derry—It has Its children to hatch and nurture. Derry has become Its true haunt ("a feeding place for animals," plural), and the place where Its children will feed. Its bond to Its children is the perfect weapon to use against It. And It believes It can't be killed—even more reason that It will stand and fight (and lose) instead of running away and getting away. It is at Its moment of greatest weakness, Its moment of greatest fatness and complacency. So now is the perfect time. Any sooner would be too soon; any later would be too late (It's children will hatch, feed and possibly branch out. Imagine hundreds, thousands, millions of Its. The entire world is their haunt. Goodbye, human race).
It's only in the book, but Beverly's father chases her all over town when he means to finally kill her. But think about it; he's a man in his forties who works a fairly exhausting job, where does a man like that get the energy to chase a healthy, fairly athletic twelve-year-old? From IT. Not only does IT crawl in behind the eyes of people you care about, it makes them stronger.