Why do ad champs get better resistances than ap champs?

I'm talking about the damage resistance items.

AD:

AP:

I left out the ability power MR items because those don't really affect ad champs.

So now if we look at these in a fight. A graves might come at me with a build that looks like this

Graves:

and an AP champ might have something like this

AP: (seems like the boots has been the go to pick because of that magic pen)

This is not a "QQ NERF AD CHAMPS" thread. It's simply a question that I've had because frankly ad champs shit on ap champs because of the crit,lethality, lifesteal, magic resistance, and stealth. Unless you're playing .