There is no remaining question of why except to those with their ears plugged.
That Warrior's sustain is merely "weirdly/worryingly high" outside of dungeons but "stupidly/brokenly high" in dungeons comes down simply to the target-count-scaling of EW Bloodwhetting and Nascent Flash's means of sustain.
This has been explained to you ad nauseum by now.
Bloodbath does not heal for its single-target value per enemy struck. It simply heals based on whatever damage you can put out. Which is exactly how Nascent Flash used to work, making it interact far better with the rest of Warrior's kit.
EW Bloodwhetting and Nascent, on the other hand, heal for the same amounts they would in single-target, but per target. Which is not how AoE works in any other area of the game.
That question has long, long, long since been answered. You just have to read any of the many hundred explanations of how and why that have been made since EW launch.