Because people who identify themselves as PC gamers do so because they believe in the inherent superiority of the PC, while those who identify themselves as console gamers believe the same of their respective console.
Its less an issue of "People who play games on computers vs people who play games on consoles" and more an issue of "People who are proud to play games on computers vs people who are proud to play games on consoles."
Personally, I play games on PC, X-Box, and Wii, and all are radically different if you look at them.
On PC I keep to my 80 hour RPG's I can't play in the family room without irritating the family, as well as RTS and MMO's (F2P, no more paid MMO for me). On X-Box, I have my CoD, TF2, and other games that are fun to play online with groups of diverse people that change from round to round. On the Wii I have party games like Brawl and Rock Band.
(Please note, above categorizations are generalizations, they do cross over from platform to platform)
Mostly its an issue of the console wars stepped up - many PC gamers grew up with only a PC because mummy and daddy wouldn't buy them a console because they already could play their games, so they tout its superiority.
On the other hand, many people who self-identify as console gamers didn't have a computer powerful enough to run top of the line games even on lowest settings, and thus never experienced many types of games such as MMO's that are native to the PC. Thus, they dismiss them, because they couldn't have them.
TLDR: They can, but people who don't are louder and more obnoxious and generate larger comment threads.