PC's do have such a CPU, quad core CPUs are nothing new, the Cell is either 8 cell with 1 redundant for backup incase on dies it can use the 'spare' or 7 using 6 cores... eitherway its just a multicore setup.
The main difference is the PS3s games takes advantage of a multicore system because it unequivocally knows what its dealing with. EVERY PS3 has the same hardware, which is wildly different to PC systems, theres so many possible setups and they cater to a wide market rather than being fully focused on whats best for 1.
What has proved successful on many of the best looking and most popular titles has been acheived when the developers have assigned each core a specific task, rather than having all of them doing all the jobs at once. Physics, Audio, AI... each assigned to a core, and then brought together, rather than lots of data being passed between cores for calculations.
Until every PC has similar components, theres always going to be a diverse market to develope for, and as is the main issue with the 360, the lowest common denominator is what they're limited to. If someone out there has no HDD drive, they cant release a 360 game that depends on the user dumping any content to the HDD. Because the new Forza comes on multiple discs, any user driving a car from disc 2 online will appear on some peoples game as driving a generic model because their system doesnt have disc 2's data, only whats in the disc drive.
They had to compromise the ideal solution (key data on the disc or HDD) because they cant get all the data onto 1 DVD and cannot guarantee every user has a HDD in there.
PCs are the same, and as such they're limited to that. Sometimes its nice having the best hardware money can buy, but if everything else is holding you back... what good is it?
Likewise the problem with the PS3s structure is that most game developers are familiar with the PC structure, which the 360 follows (it'd be stupid not to when its a microsoft product and you influence a significant share of the market), but because the PS3 goes a more logical way which works better for a unique model, rather than a wide range it doesnt need to cater to. The problem is it isnt the PC route developers know, so they whine and moan a bit, completely ignoring the fact that never in the history of console gaming has it really been any different. The xbox is a mini-PC, that made their life easier, nothing else changed... except the developers now have an lazy attitude to learning now they have the option of not having to.
PC's can do the above no problem, but with so many variations, compromises have to be made, and that means lowering the benchmark to allow it.