Usenet and the message boards being referred to are 'proto-internet' services. Think BBS, where your computer dialed into a service, and you could interact with that builiten board, the messages and users on it, as well as any files it had available for download.
Usenet had newsgroups that were very diverse and specific, and originally were just like message boards, but at some point, the major remaining Usenet servers started just sharing to each other, or maybe more appropriately, they would reference each other.
As someone mentioned before, it's a protocol just like HTTP. There's a bunch of servers all hosting webpages made in hypertext, and we just jump between them with links. Likewise, there's a bunch of servers out there hosting newsgroups, but you have to find a gateway to get started. The reason there's no 'one' company is akin to asking why all websites aren't hosted/owned by one company.
If anything... It's kinda like lemmy/fediverse stuff. You make an account with one instance, but since the protocols are the same, you can use your account on that one instance to talk to the whole fediverse network, multiple instances.
Why it costs is because at this point, it's an archive. A huge archive, of not just text discussions, but also all the files that have been posted since a very long time ago. And just like the currently 'free' archive.org, it costs money to host all of that. Usenet is a bit less resource intensive than a modern website, so it can just basically sit... But they just ask that you pay to access it, pay to have an account. In this case, you're paying to access a network that is separated from the rest of the internet at large.