Losing Boss loot

Boss loot despawing on death is completely unnecessary. Didn't think this would even be an issue until I did the first boss on act 2, only to have a screen full of mobs rush in and kill me. Can you explain to me how this is good, fun or even fair game design? Do you want to make people angry since loot is like the main driving force in these games.
I've seen instances of people dying to things like dot etc too.
Why allow players to be killed without looting a boss.