Comment by gridspy

Because if you run the simulation at a different frame-rate from the rendering it is a huge amount more work. Suddenly you have to start marshaling data through intermediate data structures and apply interpolation to everything.

If you then try to run the simulation in parallel with the rendering (rather than between some frames) it is even more work, since inter-thread communication is hard.

This stuff might seem easy for very good programmers, however on a game you want to hire a wide range of programmer skill and "game-play programmers" tend to be weaker on the pure programming front (their skills lie elsewhere)