Class Specific Items

I'm just curious as to why there are none of these except for a few occasions on champions, like Viktor's hexcore and maybe Cass who cannot build boots?

I feel as if this was implemented it would greatly improve the stability and balance of the game and make the balance team's job easier to do as they can then work with items that are for Assassins, Marksman, Supports, etc. Riot always seems to talk about how they have to get rid of some items or nerf them because of the synergy with specific champions, wouldn't class specific items alleviate most of this issue?

They create items for Assassins, Marksmen and Fighters build them. They create Tank specific items and Assassins build them. You get the picture.

Can someone explain?