Here is the thing - DPS are free to disengage, if they are feeling threatened. Tank can’t do that, or their team dies. Support can’t do that either, or tank dies.
2 roles for taking control of objective, 1 for eliminating enemy control of said objective.
Otherwise that role would be too strong.
Overwatch is a mess, because Blizzard tries to solve nearly impossible puzzle - how to make DPS center of everything, if they are mainly source of kills.