Because, after all, they'd need to be "fed", and wouldn't produce any net energy. That was in the original screenplay. But they decided that most people just wouldn't get it. So we got what we got.
Stuff like that ruins (for me) most films that ~rely on ~hard science. Explosions, for example. They are typically far too slow, with far too much dancing flame. In reality, they're virtually instantaneous at scales much less than kilometers. Saving Private Ryan is a notable exception.