Well, I don't think you can just look at which items they get without considering the differences between AD and AP champs. That's why I bring up the fact that AD can crit and scales autos and lifesteals (much better than AP spell vamps). Besides the obvious statement that "different is different", there's actually some practical reasons I bring these up: Have you ever tried laning against an AD champ once they have hydra? You just can't win anymore. You might not lose, but you're not winning - not in sustain, not in cs, not in kill potential (if they don't volunteer to die). Likewise, it's a ton easier to cs as an AD champion as any AD purchases make your lasthitting easier - AP is more reliant on spell bases and has to use resource to farm more often than ad champs do. Lifesteal I'll touch on in a bit, but it's a similar argument to the above two. Ofc, that was a mouthful, and I thought it'd be clear enough if I just showed that your argument works both ways - AP has some items AD would like, while AD has some items AP doesn't get.
Now for the reasons spell vamp's bad:
**1) All the completed items suck ** - By "all", I mean the 2, one of which is a hybrid item anyway, and the other is only viable on 2 or 3 champs.
**2) Casters don't usually need 'burst heal' **- Bring up spell vamp and someone always mentions the horror of 'burst healing', which is a farce because the champs who can burst don't need the healing, and those who need the healing can't burst (especially when the sustain is most useful - in lane). Also, as always, remember that spell vamp is gated by cooldowns and the 1/3 AOE rule, while life steal has no such gates.
3) Those champs who do use the stat well get dumped on - Immobile, sustained damage champs don't see very much play for a good reason - because league of mobility+burst makes them irrelevant. So while Morde/Vlad might be good users, they're not a good choice in general. Said another way, come lategame, your healing probably doesn't matter much if you're dead before you get to use it (walking up to someone as Vlad or Morde).