I don't think there is a satisfying explanation in the movie, it wasn't really the point of the film to give one either.

But i think there is one that's would have been a good fit.

The world getting empty of resources and our planet's condition worsening, we could have made the simulation for ourselves.

Our brain could be fueled by a renewable enough energy and creating all the comfort of modern society inside of the simulation.

That would have been a better plot for the following films too, trying to understand what was Asimov type of rules we put into the AI and how to hack it.