Because every time a PS3 game comes along that has my interest it has a Wii or PC version. To be fair the same is true of the 360:
The Orange Box
Gears of War
Bioshock
Mass Effect
UT3 - Lost interest after I played the demo
Saint's Row 2
GTA IV - GTA games always get a PC version and we will likely get the DLC as well)
Oblivion
Splinter Cell Double Agent
CoD4
Assassin's Creed
Kane & Lynch
DMC4
Overlord
Those are just the ones I wanted that are on PC =P
Wii + PC = Awesome Combo
There are only a handful of games that are exclusive to the two HD consoles that I have a any interest in. But not enough to justify a console purchase yet. If I do get another console it will likely be a PS3 since the PC overlaps with the 360 games I want much more.
Ironically people complain about not being able to play PC on the couch but now with my new 46" 1080P LCD I have an HDMI cable and optical audio running to my TV and sound system so I can play my PC games in their full glory from the comfort of my couch...even better I can use the Wiimote as a mouse and it works pretty well in most FPS games so far (still need the keyboard, which I have a wireless keyboard so np) but I have a nice hybrid setup with the best of both worlds in pretty much every way.
Long story short...I'm currently quite satisfied with my gaming arrangements.