Here's some of the possible reasons I can think of as a PC gamer (some or perhaps even most have already been mentioned, but I will repeat them anyway):
* PC gamers feel left out/ignored by both the media, the game developers and other gamers. People rarely have "XB 360 vs PS 3 vs PC" discussions. The PC doesn't even seem to be an option for most console gamers and I think that this upsets PC gamers a little bit. So perhaps PC gamers want to "save" the console gamers and inform them about this "unknown" platform? Or at least make sure that the PC doesn't drift away completly into oblivion.
* Some or perhaps even many PC gamers feel that console gaming destroyed the quality of many games being produced. That many games started being produced for consoles primarily and for the PC secondarily resulting in poorly made PC-ports.
* Many of the PC gamers are probably old time gamers that were gamers long before gaming became as accepted and mainstream as it is today. So many are probably a bit miffed that their "club" isn't so "exclusive" or "special" any longer...
* I wouldn't be surprised if PC gamers have a larger number of technically skilled people among their numbers. Which is why they might have a harder time understanding why someone would choose a platform that's a bit easier to maintain.