It's a matter of cost. One of my friends has an Athlon64 machine with SLI Nvidia cards, and the works... total cost $2500 to put it together himself.
I'll just buy both the 360, and PS3 instead. I can't keep up with PC gaming anymore, and if BF2 plays even remotely as well on the consoles as his PC, I'm sold.
PC gaming is for the elite. The 360 seems like an absolute STEAL in comparison.