Riot's current itemization is more friendly to a better version of spellvamp than lifesteal right now, and has been for a while.
I'd like to bring up this item. 
Its giant slayer passive works not only on tanks, but also most other roles. Most adc items completely and totally lack hp. They can get away with healing off a sixth of their health with every hit on high attack speed because they can die in three spells or autos from another adc.
You could argue they try to mix tank and lifesteal items, but tell that to Aatrox, who isn't very good right now. The more tanky they get, the less damage they do. It's not like you can't build defenses or penetration as a mage, or try to lock them down with cc. There is counterplay here.
Now, compared to lifesteal, spellvamp or an alternative would never be as universal. No matter how it was made, it would always be better for some champions than others. What this boils down to is that only a select few champions can use it. Even now that Vladimir's synergy with spellvamp is worse, buffing it would probably only let it be viable on Vladimir, Swain, maybe the gunblade users, and that's it. If it's a good buy on, say, Lux, it would almost certainly be broken on Vladimir/whatever.
Throw this on top of the fact that mages have much more health options than lifesteal damage-dealers. After botrk/bloodthirster/Ravenous Hydra, what do they build then?
?
?
?
?
Meanwhile, mages have options like 







The game is balanced around squishy glass cannon adcs being able to heal up damage so they don't need to constantly recall to have presence and stay relevant. Giving that to a mage, which is already supposed to have its own perks (generally either range, cc or escapes) upsets this balance. Just imagine if Azir could play like Illaoi. That would be some messed up shit.
To give another way of putting it, there are three Support item lines:
,
and
. The first provides AP and mana regen, the second provides health and mana regen, and the third provides health and health regen. Why is there no AD gp10 Support item? Why does the most AD-centric Supportive item,
, provide AP?
Because a gp10 AD item would be broken, easily outclassing Cull. If it wasn't, it probably wouldn't be good enough to call an AD Support "meta" or "not bad" in the first place, making it a noob trap. Riot thus can't support AD Supports. They don't want to give the idea it's an option when it's such a bad one, and they aren't prepared to test the effects of AD Supports being good on the meta. A good spellvamp item is the same as this good AD Support item.
So please, stop whining about balance matters you know jack about and calling people names just because you or your team fed a champion you can't be bothered to try countering.