Why can't AD Casters have good late-game scaling?

I was wondering in particular the possibility of a Rabadon physical counterpart, that gave something like +20% bonus AD and (in my mind this would balance it) had a passive that prevented regular critical hits (while allowing the likes of Pantheon, Shaco, Fiora and similar folk to crit as usual with their spells).

Plus, I wondered why mana itemization on AD champs is so lucklaster. Manamune offers an autoattack oriented boost (whereas Archangel's Staff's shield is useful to anyone) and is mid-tier in terms of offensive stats (it gives 65~ AD on average whereas Seraph's Angel offers the most AP out of any current AP item except Rabadon). Essence Reaver begins offering mana sustain too late in the game and is very lame if you are running a low AS build/champ. Imho it could be something like Will of the ancients where it says "Your abilities' physical damage sustain you for 10% as hp and 7.5% as mana, doubled vs champs" (33% effectiveness for AoE, resistances taken into account). Of course these are just numbers thrown off with no claim of balance professionalism.

I find that of AD Casters to be a role without a proper balance from ages. They are forced to eat the leftovers of other roles' stuff, the brutalizer castration being the last snub.