I had a dig around because that’s a good question.
The answer is that at high level there’s too many skills and effects going off from everything else you have access to already and the devs figured that having items buff skills that are already there instead of granting new skills is a good way to reduce that.
Yes, Epics and Legendaries have fixed abilities, with some vary rare exceptions in Conduits and Badge of Mastery. And crafting bonuses I guess, which are usually a single resistance or a couple of skill points.
Rare (green) items can have different stats based on their prefix and suffix.