My theory is that once a life form gets advanced enough, their brains start penetrating a hidden dimension that we know nothing about. This dimension is so much richer than "our universe", that they never come back to this one.
It may have started to happen to us already -- I'm sure there are a few people here who have gotten lost in the World of Warcraft dimension for a time.
true - now think if WOW was a trillion times more immersive.