I think it's just a matter of human nature- As many gamers won't play SciFi or Fantasy, but they'll play sports games, many people just aren't entertained by playing computer games. (This is where I have a dig and blame their lack of imagination

) It's just a fact. I like squash, I don't like golf, I don't want to like golf, I don't want to ever like watching or playing golf.
The other factor is complexity, and I believe it also explains the 30+ demographic. We all grew up with fun, fast easy to learn games - pacman, space invaders, moving on to double dragon, golden Axe etc etc. One stick, 3 buttons, and blisters.
I compare that to says FIFA 2006 on the Xbox (guys at my office play) I can't get into it and will never be able to compete because:
1. I don't know intricate details of the teams and players
2. I don't have hours to spend configuring a team
3. I don't know half the buttons/don't have the finger muscle memory
That and the fact that no middleclass schoolkid can afford R400-500 a month for new PS2 games. They'll rent, borrow or pirate.
Thinking about it I'll also blame culture. I look at my folks and no-one their generation plays "games" of any sort,even board games. Once you hit 18-21 you stopped playing monopoly, go-fish and started playing poker. In the same way, they look down at video games as exactly that.