Riot, what's with the aversion to flat on-hit physical damage?

There is nothing in the game currently aside from 1 mastery that only affects minions that does flat on-hit physical damage. Not one thing.

I just want to know, what's with the aversion to it?

I went looking through the champion and item list, and literally all flat consistent on-hit damage is magic or true.

List of champions and items: Wit's End, Teemo, Irelia, Nashor's, Varus, Devourer Enchantment, Cho, Spellblade mastery, Kayle, Lulu, Akali, Corki, Thresh, Warwick, and Volibear.

Not only is that the list of champions with flat, consistent on-hit magic or true damage, that's the list of every champion, mastery, and item with flat on-hit damage period.

Do you feel it would be imbalanced? If so, why, and on which scale of over or underpowered?

If it's thought to be overpowered, why? Because of life steal? Because the number for it to be noticeable over just building AD would have to be unreasonably high?

If it's thought to be underpowered, why? Because it doesn't interact with critical strike? Because it wouldn't hurt towers?

I don't get it honestly. Why is there none in the entire game Riot?