Yes one server could be bought, but there are thousands of servers with new ones opening all the time.
Anyone can start their own server starting from about $6 a month (including all tech stuff done for you).
If some evil corporation bought out a server, that server's users would move to a different server.
The way the Fediverse is structured means that it can't be bought out. The software it runs on is licensed in a way that cannot be controlled by corporations.