ive always have thought it would be cool to have mana steal items for our casters to increase sustainability or counter other casters just an idea

Mana steal, as in just giving you mana for the damage you deal, would be too powerful on certain champions. It's an ability that is designed within champions' respective kits, rather than an item available to everyone.

And if you were thinking of it actually stealing your opponent's Mana, that would be even worse.

Read up on the history of Wit's end and you will learn the first reason why this is not going to happen.

great idea Wits end was way better before