Though I've seen a few messages mentioning a low FPS in GTA 4, this is the first thread I've seen that actually ask why are the FPS in some console games lower on the PC?
It's a good question. I know all you console fanboys are going to tell us that your lovely consoles are just better gaming systems, but actually that's not true. Go down the specifications of any console and what you've got there is just a cheap miniature PC. An up-to-date PC will have a video card capable of pushing out way more pixels than a console will, especially a year or so after that console's release.
So I'm left thinking the answer is simply this:
The developers aren't willing to optimize their engines to handle PC hardware.
Why? Well, if you want to humor a conspiracy theory, it's because PC piracy rate is off the hook, more copies of games are being pirated than sold, and the developers don't want to invest much time or effort into that market. They put just about as much effort into the PC port as they think people are willing to pay for their games: slim to none.