Because all you're doing is wasting people's time. You aren't making the dungeon go any faster by flexing your WAR or Tank Muscles by self-sustaining from 50% to 0%. Nobody cares. It's faster to wipe and start again if that many people are dead before 5% or even 10%.

When people do this, they detract from people's gameplay and might as well just have not played the fight at all - and I doubt SE will do anything about it considering they're dead set on making Tanks Invincible(unless you're DRK).