Indeed, forums are almost gone. In particular, I miss one forum about science fiction, one about aeromodelism, one about electric vehicles (another still exists) and one about anarchism. An interesting hold-out in the country where I live, is a military forum. Mods do a cursory background check, rules say that respectful discussion is the only kind of discussion accepted - ironically, the military forum has a peaceful atmosphere. But it could come crashing down much easier than a social media company.

As for why forums disappeared - I think that people became too convenient. They wanted zero expense (hosting a forum incurs some expenses and needs a bit of time and attention), and wanted all their discussion on one place. Advertisers wanted a place where masses could be manipulated. Social media companies wanted people to interact more (read: pick more fights) and see more ads - and built their environments accordingly. Not for the public good.