Everything in the game is balanced around the biggest abuse cases. Look at old spell vamp. It was balanced around the abusers. Steraks? Liandrys? Even current on-hits are balanced around those who can proc them the most and are put at an appropriate damage level for both the scaling aspects and maximum use cases.
There's a reason it's "old spell vamp", because they couldn't balance it around the abuse cases and normal use cases. It was either busted on the abuse cases or not worth it for every other AP champion. Sterak's was similarly nerfed hard because of the abuse by Marksmen, which has resulted in Riot testing out melee only effects on it.
Liandry's and Infinity Edge hit the sweet spots where even if you're not getting the maximum out of the item it's still good, and it's just a bit better on those who can hit the condition.
Crit Damage differs from On Hit because of the stacking of different effects. BORK's magic damage is improved by Wit's End MR steal, the stacking of Rageblade makes all other On Hit effects stronger, etc. Additional Crit only makes Crit more consistent (and has diminishing returns at a point, the difference between 80% Crit and 100% Crit isn't much), it doesn't multiply the power of other Crit items.
TLDR: they'd have to pick some AS value to balance all the On Hit items at, which would be based on the abuse cases (much like Spellvamp was), which would make the builds feel weaker to other champions than they do now.