Why can I run a Pc game at 100% but cant run a mame golf game? 
why do I need an overclocked 5hz CPU to play wii games, when the Wii has a crappy 750 mhz CPU?!?!
It's because you are asking a computer with a simple instruction set (PC), to do something a complex instruction set computer does (GT hardware). A complex instruction CPU has custom instructions to do multiple complex operations by just saying one command. (like load this data from this memory spot, do some math to it... and then write it back.) This command can take several times more cycles to complete on a SI CPU not including the wasted cycles waiting for a slower bus.
this is what an emulator does. it takes those simple commands and blows them out into their individual commands and applies those individual commands to the CPU it it native language to get the work done.
it's like having a mould for bricks to make a castle. the GT compete computer has a mould that does whole walls at a time....A PC has a mould that does one brick. so you have to mould one brick many many times over to build the castle. thus needing a crapton of horsepower to make up for the many many times more operations you must do to accomplish the same task.
in a nutshell, the game is highly optimised for the hardware it runs on. trying to convert it on the fly to run on some other random hardware takes a pile of extra work. The advantage of Complex instruction set CPUs being, that you don't need a super expensive high speed CPU to make your game work. The disadvantage being, that you are limited by what your CPU can actually do without redesigning the CPU with additional commands.