I believe GPU would be more expensive, it would increase human labor too much. CPU is universal and has more RAM. Optimizing very sophisticated ray tracing fo

I believe GPU would be more expensive, it would increase human labor too much. CPU is universal and has more RAM. Optimizing very sophisticated ray tracing for GPU with tiny amount of VRAM could be very tricky. It is easier to do dirty code on CPU, and wait for couple of weeks. Apple also had spare CPU cycles. This stuff could run overnight in background.