So you really haven't played an MMO before. Let me break it down for you:
Tanks burn out.
Healers burn out.
The entire group is dependent on tanks and healers being on the ball all the time. You can get away with lackluster DPS, but you always need competent tanks and healers, and tanking and healing are largely thankless tasks - both in not being able to see epeen-enlarging big numbers and in having to suffer the rudeness of other players: If DPS dies? Eh, screw that guy. If the tank dies? Everybody dies.
Since tanks are so more important than their compatriots, competent tanking is in very high demand, leading good tanks to end up tanking full-time due to the absolute necessity for having them around in games that revolve around the holy trinity. If they're hybrid classes, now they're not. If this is not their preferred playstyle, they eventually quit. If this is their preferred playstyle, they burn out anyway because everything revolves around them.
This should have been patently obvious for anybody who has ever run a dungeon in any MMO in the last ten years, because you will always fill up the DPS slots nigh-instantaneously, and then end up waiting forever for a tank - longer if you want a competent one.
It's not just PuGs, either. The most important player in the raid guild is the main tank. If he leaves, you're screwed. You're all screwed. DPS can be replaced at the drop of a hat: Not so for tanks. And since he has to have a perfect attendance rating, guess who's going to burn out first?