There are innovative, sandbox MMORPGs out there right now-- you just don't play them.
The risk takers are, just as in movies, found among the smaller, more independent studios that build and maintain their games on a tiny fraction of the funds that places like Bioware or Blizzard spend.
Of course, their graphics are much less impressive. Their marketing is almost nonexistant. But they are out there, and if you really wanted to see innovation, you'd be playing one of those games.
When movies and games cost hundreds of millions of dollars to produce, you really can't fault the big studios for endlessly trying to reproduce the last big budget hit.