Some people don't have very good income, and can not buy a good gaming PC.
Wait a sec...you're saying that everyone doesn't have a multi-thousand dollar beast of a machine to play games on? I totally agree with your statement, by the way. I think income is a major factor in the choice for which gaming platform(s) a person uses.
For instance, I have a gaming PC because Ive been accustomed to playing games on one since long before the newest gen consoles came out. Once they came out, I got an Xbox and PS (skipped the Wii). Haha, actually, initially I was going to get a Wii because there were a few games on it that I wanted to play, but with Nintendo's inept launch, I couldn't find one anywhere, so I got frustrated and got an Xbox360 instead. With the Xbox360 and PS3, I no longer feel the need to get a Wii. Oh well, Nintendo's loss. But anyway, if I was a bit more limited in my finances, I would've gotten a console instead of an expensive computer.
Even ignoring the income issue, many people will buy a cheapy computer for just web-browsing or email or downloading porm or something. And then buy a console on top of that for their gaming needs. It's easier for a parent to just buy their children a console for entertainment as well...more cost effective.
And I still think the whole platform wars thing I see on these forums is stupid, and both sides are guilty of contributing to it and drawing it out ad infinitum. I really don't understand the immature need for a person to feel better about themselves by putting someone else down.