I'm guessing one of two things may be happening
(1) console games for the PS2-3/360 are getting sloppily and quickly ported back to the PC, without taking the time to make them efficient for the PC architecture.
(2) video card / cpu companies are somehow encouraging this mess in order to drive sales of new computers, in a time when sales are slumping because machines are already ridiculously over-powered for the average consumer.
just an idea. i'd love to see more games ported WELL to the PC. a criterion collection or Mobile High Fidelity for the PC gaming industry.