Why is life steal so OP?

I been wondering, why life steal can be purchased to the point that a person becomes a monster due to the heals he/she gets from the life steal. Compare that to the spell vamp, you can not (afaik) get 40-50% spell vamp, while you can stack life steal up beyond that even if you wish and you do not have to sacrifice any significant damage to get that much life steal. why is that? why is not spell vamp and life steal equal? due to range of spell vamp? can't be cause ranged champions like vayne rule when they get high enough life steal. So my question is several... 1. why is life steal so OP? 2. why is not spell vamp and life steal equal? you certainly can't buy spell vamp stuff like you can do life steal 3. why do life steal items do not have save energy drain for example for ranged mages etc that uses spell vamp. that has to use mana to get their result while life steal hits on auto attacks for example.
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