Why not give Kassadin MR/level?

He's an anti-mage with no magic resistance. Several assassins have scaling MR including Zed and Talon, this would be a decent buff to Kassadin so he's not a worthless laner while also not making him into the pre-nerf killing machine that he once was.

Edit: Yes I'm aware his passive gives his magic damage reduction or else I wouldn't have made a thread on him. His passive basically gives him that ~25 MR that other champions have simply in their base stats. My point is that he's wasting a passive to be an anti-mage and yet he's no more of an anti-mage than other champions with scaling MR.