It's not so much the software that they need to mimic but more so the hardware. They have to emulate the hardware to be able to run the instructions on a differ

It's not so much the software that they need to mimic but more so the hardware. They have to emulate the hardware to be able to run the instructions on a different cpu/gpu/audio architecture than what the game was built for. That's why it takes a much more powerful system to emulate a much weaker system.