I have flora as an example of an ad caster that the others could hope to be, she has mana but seems to not use any unless she's struggling in lane. But champs like talon hecarim and jayce can run out of mana easily, winning or losing lane. Other successful ad casters like zed yasuo and riven don't have mana at all. Yes zed has energy instead, energy is balanced because it has a high %regen rate compared to mana, but it has a small capacity. You won't build manamune on a talon or hec, but it's great on jayce, same with essence reaver. Hec can build it effectively but it's really only viable in his more damage focused builds, rather than the typical iron horse build.
You could try arguing that not many ap items have health regen but most ap items are made primarily for mages over mage fighters like Diana or akali. But akali has insane regen with her passive, Diana has an ap scaling shield, and even fizz has many tricks to avoid damage. So it's not really needed.
I just feel like ad champs should have a simple item that both provides a decent amount of ad for their skill scalings as well as a small +50% or +100% base mana regen and maybe some cdr, but thats not really needed, there are plenty of ad items with cdr already