I'd imagine they won't do this because it's going to lead to hostile behavior in the case of a wipe or very new player. It's also not going to be fun if you're the good player in the group and the other 3 players result in you losing all rewards for the dungeon.
That said, I wouldn't mind some form of reward for good play. Comms just don't work, the majority of players either don't care or aren't experienced enough to judge who deserves one. FF14 unfortunately is one of the most thankless games to perform well in. It gets you nothing but occasional belittlement.
A system that evaluated you personally based on dps, healing (minus overheal), amount of damage mitigated and so on and gave you something, even if only some collectible stars that lead to achievements with a few rewards for different tiers would be a big improvement. It would give newer players motivation to try and improve. But you don't lose on anything significant should one player slip up or run a bit slower.