That's trinity design for you.

I mean, they could make content where tanks and healers aren't needed, just remove the unavoidable damage. But then tanks and healers would become pointless. You need to force them into groups via content design so they can viably live out their niché fantasies, which in turn means other people have to suffer in queues for their sake.

And as humans are, said players then aren't grateful for being granted viability at the cost of other people, no. They demand further rewards as compensation. You won't find this kind of madness in books, only real life.