My guess is that the goal of the simulation is to model Earth in the 20th & 21st centuries. So say they've got resources set aside for N such Earth simulations. They can run N simulations one after the other, halting each before sub-simulations become possible, or they can allow an N-deep stack of simulations to form. The problem with the second option is that the simulations will become less-and-less accurate representations of the 20th/21st century as you go down the stack. Copies of copies of copies. Also, you'll need to allocate memory for all N simulations to run at once, whereas with the first option you only need memory for 1 simulation at a time.

On the other hand, I'm sure they/we will run many different kinds of simulations, so the question really is: of the kinds that contain conscious people living in the 21st century, which is most common?