What does AWS gain by improving IAM? There are barely any competitors, so they won't be losing people for that. They offer their own AWS professional services happy to charge you for making it "understandable". Their service agreements largely absolve them of client mistakes. Which usually result in larger bills from AWS.
AWS is a ball of complexity because it grew organically that way, and they don't have a culture of explaining, or, keeping things simple.
Both of those things would require strong strategic guidance, and a real effort to do.
Unless Bezos edicts: "Our APIs must remain simple even as they scale, and we must document in a manner that keeps the 80% common path easy to use, while the remaining 20% arcane functionality available ..." then it would happen.
But it won't.
It's reasonably well curated arbitrary complexity, it is what it is.
This is not an issue anyone handles well.