Think of it this way..
Pick 10 different cars, even some from the same manufacturer and you will find parts in one car that cannot be used in the other.
Things are made different, different sizes, different ways of functioning, different connections, etc.. Just like buying an air filter for your car. Have to get the right model, made to work with your car..
PC's are the same way.. Just like you can't run Apple software on an Intel/AMD based system (Apple now sells PCs that use Intel CPU's so its now possible to do both, only on those specific Apple computers). Their chips are built differently, process data differently, and communicate differently. They can't talk to software coded for another platform.
Just like you can't take an NES game and run it on a PS3, without an emulator.. It's all different hardware. Just isn't gonna work.
PC's emulate other systems by software. A programmer emulates the function of the console CPU/GPU and other hardware, via a software platform. This takes enemormous processing power, even when you have plugins that allow you to use your own graphics card for 3D acceleration. It has to be tweaked to be extremely efficient, and accurate. Which is why emulators take years to develop into a truly stable and playable product.
I'm sure it is possible with an Intel Quad Core to maybe get a rudimentary PS3 emulator going, but it won't be near powerful enough to actually play games or anything like that.. The programmer would also have to be experienced in programming multi-threaded, multi-cpu applications, to understand how to best use the resources and distribute the workload across multiple cores.
It's really a lot of work.. It takes something like a 100Mhz 486 to fully emulate an NES system, running at a puny 1 or 2 Mhz.
On top of all that, Emulators are developed using a process called Reverse Engineering. R.E is an extremely expensive and time consuming process, and that is for corporations that actually employ people to do it and have millions of dollars.. Emulator programmers have to emulate the hardware using NO original code from the actual system because otherwise it would be copyright infringement. So they have to design their software functions, design an OS/BIOS system that communicates with the hardware, allows the hardware to communicate with each other, handle input, output, and understands the language the games are written in, and then run it. Using NO original code. So it takes a long time and dedicated programmers to do this