I am not sure that is true. What other than DRM stops people from setting up their own competing servers? What other than a lack of incentive for the game makers prevents community-run servers from interoperating, creating an enormous peer-to-peer universe?
There is no DRM in an MMO. What stops you is the lack of them giving you the server software.
Facebook doesn't give you their server software either, would you call that DRM?
This is absolutely no different. Considering that a huge proportion of Hacker News is devoted to web based startups, you would think that this would be fairly obvious.
Thus explaining why people need to do things like patch the game and/or modify their hosts file in order to connect to third-party servers. If trying to authenticate the server you connect to and leaving you with no options for connecting to third party servers is not DRM, what would you call it?
"What stops you is the lack of them giving you the server software."
Nope, the protocols are reverse engineered and people create their own server software.
Which is one of two reasons (the other being anti-cheating) that newer games withhold a large part of the game logic on the server side. It is possible to reverse engineer game logic but impractical: you might as well rewrite the game.
If you have a look at efforts to reverse engineer Diablo 3, you'll see that most of the game is missing without the server. You can run around but you can't do anything.