> To host RSS you just need a domain name and a server. It's pretty hard to stop somebody from getting those.
The word "just" is doing some heavy lifting here. Even cheapo web hosting (that automatically handles TLS) and a domain name still costs more than the "free" of Facebook/Twitter/etc. That's besides the non-zero technical competence to set it up and then actually update the RSS.
Then you're at the mercy of the host's AUP/EULA. You can have your account terminated for a number of reasons, like getting DDoSed because someone doesn't like what you said or your account is hacked and is used to host malware.
Most people, even the technically inclined, don't necessarily want to be their own system administrator. It's an order of magnitude more effort than signing up for a centralized platform and pressing a button. RSS might be fined for a certain demographic but it's completely untenable for the majority.
That is besides the ergonomics of RSS. Because RSS readers want to display item titles because they treat all items like more formal news articles they de facto require posts to have titles. Common blogging platforms also want titles on posts.
Short form content is attractive to many people because it doesn't require formalities like titles or subjects. Short form platforms even understand inline tags (hashtags etc) so posts have even less friction then the typical blog software.
RSS, because of common patterns of readers and generators, sucks for carrying short form content. To make it work you'd need not just generators to make items without titles but also add a "short form" mode to feed readers that don't try to make every feed item look like a long form blog post. Both sides of the equation would also need to capture things like references to posts to display posts as replies to others.
It's all doable but requires agreement on what changes need to be made to the format/schema then implement those in feed readers and generating software. The Atom format tried some of this but too few bits of software really leveraged the format.
I say all this as someone that loves the idea of RSS. The lack of affordance to short form message-like content is a major limitation of the format and a design oversight. I think social media would be very different today if RSS (as an ecosystem) handled informal short form content better.