Pure REST API is a huge pain in the ass to use and leads to bad performance because of waterfall requests. It is like the number one thing GraphQL solves for most people, decide which nested relationships to inline into a response.

The simplicity makes it bad to use, if you work around the simplicity (arbitrary inlining, extra fields to choose what to inline, etc) then you end up with non-standard complexity.

Bad performance if you haven’t heard about HTTP caching. REST implies cacheabality.

I assume, it's less about the complexity and more about most people not understanding REST.