Unlike all AD that mostly goes for BT
Mages need an item like BT

BT is cheap IMO

I think spell vamp has always been relatively inefficient sustain on burst champions, but good sustain on dps ap champs. I do more or less agree that having something that gives at least 60 ap with spell vamp would be good for mages though, since some certain champions (looking at vlad and morde especially) tend to essentially require spell vamp to be effective, and nerfing the viability of spell vamp items hurts the viability of these champions.

Like Will of the Ancients?

Spell Vampirisim isn't seen very often on mages because it has questionable utility on them. "Mage" tends to imply "burst caster", and said burst tends to be unloaded when you're at full health due to either initiating the fight yourself with some widespread crowd-control or directly fallowing up an initiation. Basically, restoring health isn't useful when you're at full health.

Life Steal is more useful on carries because they deal sustained damage. You have to pew-pew-pew-pew rather than shoop-da-woop to kill things. Or basically, sustain through a fight so you can deal sustained damage. With burst you only need ~2 seconds to accomplish things.

Spell Vampirisim is incredibly useful on AP scaling champions that deal sustained damage though - such as Rumble, Vladimir, Mordekaiser, Cho'Gath, Jax, and Amumu. The main thing here is such champions are tagged with "fighter", and tend to be unusually durable. Mordekaiser and Vladimir are so good at utalizing the stat that Spell Vampirisim had to be changed to a unique, because they were better carries than actual carries with it. Also, they could solo a 5-man team with unhealthy regularity, which didn't strike many people as balanced.