Becoz you dont need more gear when you beat the game. Just a cosmetic mount or cosmetic armor would be a good prize … NOT GEAR.
When I killed Bhaal on Hell in D2R, he just dropped a regular loot which I dont need. He didnt even drop a Rune. And I was okay.
On GR140+, I was missing an Ancient boots for my Necro and an Ancient Shoulders for my Witch Doctor. And the drops were RNG and I understand. I just quit when I realized… what’s the point? I have beaten the game already. Just move forward and play something else.