I honestly think it is because if you balance around any of the other classes in the game, it creates much more unbalanced situations. Sustained damage dealers SHOULD be the corner stone to a team composition as they are your focus for dealing damage over a long period of time, which is key for every kind of objective you could want. So you get people to protect those who are your highest sustained output, and those who try to destroy the enemy highest sustained output. This starts to branch out into bursty people, sustainy people, tanky people, protective people, etc.
I think you could make the same argument about any type of subclass of characters in a game, but in my experience, the sustained damage dealer is ALWAYS what you want to build around, whether it be to counter it, or save it.
But like others have said, they have a much slower power curve, so "balancing around them" makes them more of the pace car.