[nebulus]

I've always thought that this was the most likely reason for our simulation, and the most telling evidence for it:

In a universe where intelligent life is common throughout the galaxy, there'd be no way to tell what our species would have become left to its own devices. What discoveries we would have made, what cultures we would have created, what our destiny might have been.

The only way to discover it would be to simulate human life on a planetary scale in a universe where other intelligent life is mysteriously absent. Which is exactly the situation we find ourselves in.