It happens when you're working and have sufficient experience, knowledge and practice when it comes to web, programming and HTTP.
It's impractical, irresponsible and insane to have anyone other than a person with sufficient knowledge to work on application / API that supports OAuth and for person with proper expertise - the protocol is simple because it's literally the bare minimum needed to establish secure request/response pattern.
I work with SAML 2.0 and OAuth2/OIDC, I deal with programmers and other people within areas of API, authentication, authorization and it is true that majority of people I interacted with are having troubles understanding these protocols. However, what I can attest to is that they're also extremely underwhelming as people working in IT, with bad work habits and misplaced focus. It's impossible to explain or clear up the protocol to such audience because they lack much more when it comes to knowledge and discipline. But, I did interact with several brilliant people with far less experience compared to me who did get the purpose of this protocol and who implemented it successfully and have done so quite fast.
What the wall of text means is that where there's will - there's a way. You can't force knowledge into audience who can't or refuse to understand it, so there's no point in even trying to do it let alone draw conclusions how the protocol is hard, confusing or difficult when there's whole knowledge void in other pre-requisite areas.