The issue is that that’s only a solution for a certain type of multiplayer game. MMOs and battle royales for example cannot feasibly implement community servers as their main form of multiplayer connection, because very few people have the capabilities to host such servers, much less moderate them at such high player counts. Heck, there’s even arguments to be made for the value of public matchmaking, despite how often it gets blasted in spaces such as this. Private servers are unfortunately not a one-size-fits-all solution.
You’re right, there are, but my point was that private MMO servers are significantly harder to host and moderate than a private server in a match-based multiplayer game like say Team Fortress 2. An MMO that relies on private servers is almost certainly doomed to fail, so it must have some form of official server, which then will need some form of cheating prevention.