One, they should be programmed that way if the idea is to be taken into consideration.
Two, URF was a result of an idea with a number of unforseen consequences; besides this is not giving everyone shyvanna's dragon Q because that one is an auto attack reset + 2 autos & realistically you won't apply it everywhere unless the enemy has a lot of melee champions that let you do what you want.
Why would you build this item over a
if you're an on-hit bruiser ? If you're buying it, you're buying it on top of the botrk/
. Which is a squishy/carryish build style with clear strengths & weaknesses.
Sure, it will give on-hitters more damage late game. But it comes at the cost of a defensive item slot, something on-hit champions highly need.