They’re just not doing it where you were in the past.
If you think Facebook comes the closest to a community building site for younger generations, your information is very out of date. Young people don’t use Facebook.
Even if you go on like Reddit if you post certain things in certain subs you'll get invited to private subs talking about whatever niche topic.
Same with Discord, lot of small communities there.
Really I'd call that kind of homogenization a large part of the problem
Is the only healthy Internet one where there's 500 different platforms and to try and build a community you have to be an IT worker? How is it a bad thing that someone can hop on Discord or - indeed, as much as you might hate to admit it - Facebook and effortlessly create a group for their friends or local club?
Centralization is not the problem - it's what those companies can do with their power and data that is the underlying problem. And attempting to de-centralize things won't solve that problem - either some platforms will grow to a point where abuse happens, and/or the point of abuse will lower over time to cover those smaller platforms.
Without, for example, data privacy laws, nothing will change, whether we're on 5 platforms or 500.
(As a brief aside, since you brought up IRC in TFA... is it at all relevant that, even then, there were like 5 main networks?)