Because most people don't know how to do that. Mainstream social media has huge reach and monetisation opportunities, so that's where most people go.

What's needed isn't a nostalgic return to the 90s, blogging and all, but a completely non-corporate internet, probably using a separate set of protocols with novel reader/browser tech - self-hosted and/or distributed and/or torrented, simple enough for anyone to set up a server at home, no ads, no tracking, no corporate hosting or influence of any kind. And no "open source but impossibly complicated for normal people to use."

It doesn't have to be fast, it just has to be available with minimal friction for set-up and content management.

Let ten million private sites bloom and see what happens.