Tanking is about situational awareness and knowing when to use cooldowns.
Healing is about... I don't know, except that healers are clearly Gods of Patience, because damn, I cannot figure it out. Too stressful.
DPS is about numbers.
Clearly DPS has the lowest responsibility in low-end content in which enrage timers are soft and you're likely to be overgeared. Their gear can carry them. DPS can be complete noobs. Is this bad?
I think it's fair to say that the majority of players are somewhat lazy and uncaring. That's not a flaw of them as people, just a reflection that WoW is a game and to many people games are about a sort of mindless fun and socializing.
That leaves a minority of individuals for whom WoW is srs bns. They can be the very high end DPS who are pushing enrage timers and don't have time to overgear and in general are pushing the very edge of game possibility.
And there are those who desire responsibility. They can make tanks or healers. In a raid the ratios probably work out well, with an extreme minority of tanks, slightly more healers, and a lot of lazy DPS.