The headliner is Magnus.
Not to anyone unfamiliar with chess - they are probably more familiar with Beth Harmon then Magnus Carlsen. It's why Chess.com runs the POG Championships - because personalities like Pokimane have 9+ million followers on Twitch, compared to Hikaru's 1+ million.
I really have no idea why chess.com lets people pick their own starting rating.
Because your starting rating matters very little once you play more than a handful of games on the site - no matter what you may choose at the start, once you've played enough games, it will change very quickly change to actually reflect where your stand relative to others in the player pool.
It's the same with the rating points refunded when a cheater's account is closed - it doesn't really matter to your rating, as after paying some more games your rating will have gone back to where it should be, the points refund just helps people feel better about it.
Or why they allow speedrunning
Because Chess.com wants more people to try chess and more people to become chess players, and one of the ways to do that is when people who don't play chess sees a streamer having fun with the game in a rapid fire and entertaining format. A speedrunner account is marked as such and people whose lose to that account get their rating points back, so there's no deception involved. Maybe someone's ego might be a little bruised by getting crushed by a Hikaru Nakamura speedrunner account, but on the other hand - hey, they got to play Hikaru! When else might they get that sort of chance?
or why they don't have a separate classical rating.
Because while it's possible to play a classical time control on Chess.com (90|30 basically, yeah?), very few people do. So any rating would have the same problem as someone who starts off completely new, in that it would be wildly inaccurate as there wouldn't be enough games for it to reach a stable and accurate measure.
Your best bet would be to see if there are any clubs here of people who want to play chess at classical time controls: I suspect https://www.chess.com/club/dan-heisman-learning-center is the largest such club, anf they run regular slow chess tournaments.
But for people rated 500 on chess.com, you never know what you are gonna get. It could be someone who plays like magnus, or someone who doesn't even know how the pieces move.
I suspect that's a measure of Chess.com popularity and how someone at your level is going to encounter more of those sort of completely-new-to-Chess.com-with-no-actual-idea-about-their-actual-playing-strength people playing their very first handful of games than at a less popular web site with fewer new members at any given time.
Lichess is working for you - awesome! It is better for you - great! Lichess is aimed at people who already play chess: it's open source and its direction is governed by the people who are already involved and invested. Chess.com's strategic vision is to get more people playing chess - to grow the game overall. Hence the style of the interface and the sorts of games it promotes. Maybe some of those new people to chess will go over to lichess - that's fine. Enough will stay at Chess.com to make it worth their while to try and keep growing the game and trying to make it more accessible to younger people and to give it more visibility.
To @Ian_Rastall's earlier analogy: the Lichess movie theatre isn't showing the biggest blockbusters - maybe they'll have them on screen for a short while, but they'd much rather show the independent, art house films, and that's what they do most of the time. The Chess.com multiplex is showing the latest Marvel movie, and they'd love to find someone as bankable as Dwayne 'The Rock' Johnson to headline it (sorry, Danny, you're not him
). The people who watch independent cinema sometimes can't help but look down their nose at those plebians at the multiplexes: but those plebs are enjoying themselves and the multiplex is profitable and is even getting into making movies. Sometimes the art house folks want to 'slum it' and tell these common folk that they're so much classier, and the regular folks just look at them and think "snob" (well, I was thinking of the word that the crowd uses for Ted Lasso when he first arrives to coach soccer, which eventually becomes a term of some endearment, but I'm not sure if that would pass the auto-filter...)