Think about it. Making pre-fabricated content is way more effort than it's actually worth. It's much more cost-effective to develop the systems for players to make their own fun. "Give a man a fish and he can feed for a day" and so on.
People play the same Call of Duty maps over and over on multiplayer mode, not because they are such amazing maps, but because playing with others is. You never know what someone else is going to do.
Applying that convention to the MMO setting, you get the sandbox design philosophy. A philosophy which basically stipulates that players should have some, if not all, control over the game-world.
Of course, the downside is the developers have less control over what people do. For example, maybe some cruel person will build their house on a hilltop which was someone else's favorite vantage-point.
But the upside is that it's really hard to give meaning to MMOs without these aspects.