Two reasons:
1) Skills. Heavy use of skills. If the rogue is missing, an entire side-quest might not be very feasable. Diplolacy/Bluff are heavy. If the character that is usually the front-man isn't present, then it isn't fair to the others to have to blunder through things. Knowlege checks are important, too. If you need to research how portals work and the guy with Knowlege (Planes) is missing, you're screwed.
2) When battles do occur, they can be very deadly. If I were to not require the PC to be played most of the time, then saw a big battle coming up and require the PC to be played, then they would know. They wouldn't meta-game it because they're cool like that, but it wouldn't be nearly as fun. So, if I've got an assassin ambush coming, and I took into consideration the ranger's battle prowess for this scenario (not to mention her Spot/Listen skills) then she's going to have to be there. And, I don't want the players' surprise to be ruined.
So, a missing PC can really hurt a good game. I like to give everyone their moment to shine. If they arn't there, it can make things harder on the others. Not impossible, but I don't want to have to either change the scenario because someone isn't there, nor do I want to inconvenience the others in what was supposed to be a fun moment.
And when they die, they die, because they're getting full XP for the situations. I suppose I would relax that if they got no XP, but... well... it hasn't ever come up.
EDIT: The PC being nice in terms of doing dangerous things is mostly just in fun. We arn't putting the PC in more danger than normal. It's not like the wizard is the one opening doors, but if its a choce between the fighter and barbarian, and the barbarian's player isn't present, well, the barbarian is the one doing the door opening.