There's nothing (yet) incentivizing a clean, efficient, pleasant web, owing to the fact that scale of the stuff the web connects to/makes available doesn't have any meaningful "competition" whatsoever.
Craigslist could make a lot more money, especially in the short-term, with different design and monetization choices.
Sorry, your downvotes don't make this not true.
Look, why does google no longer have the title of "simple, no frills search?"
It's very very difficult, pretty much impossible for any new competitor that provides a useful, good, quality web-related service, to not be crowded out by incumbents and anti-competitive behavior.
It's technically trivially easy to make a new/better Uber, Doordash, Google, whatever. But they're going to throw everything at you to make you not happen.