Let me explain from a game dev pov.
Overwatch has to have asymmetric balancing.
Dmg is greatly preferred because elims is ‘fun & exciting’. If the heals are too strong, we get healwatch. Nobody wants to watch that.
Thus heals has to be lowered overall. While I agree to with you that brig is underpowered at the moment but she is very hard to balance, tweak in either way and she gets OP.