Why are tanks dicks?

Except for Titan HM, it's basically universal that groups need tanks more than tanks need groups. Tanks know this and many don't hesitate to exploit this fact.

By and large, there's no incentive for tanks to be nice to or stay in bad groups because they can get another at the drop of a hat. Since incentive drives behavior, it's not shocking that the lack of any incentive to be nice/friendly means that many tanks aren't.

Couple that with a job lacking in glamour, and it's not surprising. Tanks don't get to put up big numbers or drop clutch heals. If they're really good you basically shouldn't even know that they're present. Acting like a prick is one way to vent frustration with being taken for granted when things go well, blamed when things go wrong, and rarely ever appreciated.