It's really not that simple.
PC game developers have to develop a game that runs across multiple PC platforms with completely different setups, whereas PS3 game developers can concentrate on optimizing graphics and performance on a single set of hardware.
Take for example God of War. If you took the PC equivalent of the PS2, no way you'd get a game that looks that good. Or even Resident Evil 4. That's why multi-platform games tend not to have astounding graphics compared to exclusives and also why ports tend to suck too.
Imagine the entire Crytek team investing all of their coding and programming efforts into making Crysis work the absolute best it can on your PC, and your PC alone, they'd know exactly where the limitations are rather than guesstimating and leaving large performance/quality gaps to make up a broader range of performance compatibility.
PC still has an edge for graphics and computing power, but the fact that exclusive console games are made to accommodate only one system means that a console will last a lot longer than a PC. I mean, PS2 is 8 years old already, what did we have 8 years ago in the PC world that could compete with games like God of War or Resident Evil 4 (a port, yes, but a surprisingly well done port for the PS2), I'd like to see what a Radeon 7500 or GeForce2 is capable of today...
Also, consider the hi-def video we've seen of Killzone 2 gameplay. How many can comfortably say that it can run absolutely perfect on their specs if it was released as a PC game. All the videos have unfortunately been of that first, dark muddy looking level, but one can still see some intense volumetric lighting, a good deal of post-processing effects and some pretty complicated particle effects, plus, we know that there are ridiculous polygon counts, like each soldier being composed of as many polygons as there were in an entire level of Killzone 1, that's pretty ridiculous.
Actually, taking a look at it again, the motion blur is really well done, check out the part where he fights the big machine gun guy. I wouldn't judge texture quality or anything, the video is compressed...
And the longer a console is out, the more and more developers get used to that system and improve on their methods to produce better visuals and other game elements. PC doesn't have the same advantage.
I'm still all for PC though. I like building em and of course, you could do a lot more with them than you could do with a console.