"can't"?
There are private server instances for World of Warcraft, actually.
There are private server instances for World of Warcraft, actually.
Even so, this breaks down: this effect is (as you have noted) not just limited to MMOs. It actually applies to all games/services that use some kind of centralized network to connect players/friends. This means everything from Team Fortress 2 to Warcraft 3.
Yes, it's not as good as the real thing. Neither is running Starcraft without Battle.net, which doesn't stop it from being a fun game even in pirateable form.