Comment by lentil_soup

Because it's never that obvious, it's not really 101 of gamedev to move everything to the GPU.

You know your target specs but where the bottlenecks are and how to solve them will be constantly shifting. At some point the artists might push your lighting or maybe it's physics now, maybe it's IO or maybe networking. Which parts do you move to the GPU?

Also a GPU is not a magic bullet, it's great for parallel computation but not all problems can be solved like that. It's also painful to move memory between the CPU and the GPU and it's a limited resource can't have everything there.