Urgot W does reduced damage with on-hit effects, why couldn't phantom hits and the Kog'Maw rework?

The way it was presented, you'd think it just wasn't possible for an on-hit effect to be coded to do less damage with special autos or modifiers. Sated Devourer and Kog'Maw both got nerfed, changed, and deleted/reverted primarily because of their absurd synergy with on-hit effects. Why couldn't phantom hits or Kog's W just do reduced on-hit damage? Rageblade is also kept in a fairly weak state because of its on-hit synergy.