They are just some slight variation of the fundamental idea. For example resource policy and org SCP are just the same check on a different level (e.g. more of who has access to what). They are attached to Organization and individual resource respectively (vs Account) so they need to exist in a separate place. And then in use they are ALL checked before an access is granted.

I don't work for IAM but I worked for several other teams over the years and IAM is actually one of the least confusing services. But I am definitely biased and have more than average amount of experience on this particular subject. I still think the general idea is more sane than Azure Account for example. I do think this reflect on the philosophical level where whether cloud are building blocks or are they consulting projects. I personally think IAM is done right in that regard.