PPC OS X apps didn't need any low-level system emulation; they could only talk to hardware via the kernel, and they could only talk to the kernel via system lib

PPC OS X apps didn't need any low-level system emulation; they could only talk to hardware via the kernel, and they could only talk to the kernel via system libraries. So at kernel boundaries you could simply translate the PPC system call to x86. Whereas basically all consoles expose low-level system stuff to the game, such as memory-mapped IO, low-level GPU commands, TLBs, DMA, etc. All of this can't be emulated without a large amount of overhead. At least 6th and especially 7th generation consoles have such complex timings that emulating the clocks of different parts isn't generally needed. And it's possible that Xbox 360 and PS3 require games to go through their kernel for a lot of hardware access; I don't know.