Because the tank working harder there is how they contribute to the run. Might as well ask why I should be expected to do anything but stand next to a mob and spam bootshine. It's easier for me! Make the run go a lot slower, and be less fun, but easier for me!
Also, my own experience tanking is that I get bored with small pulls and when things aren't dangerous. It's easier but also less fun. Same with healing.
And no, there's no hard feelings.