Seems redundant to have a Express backend + REST API, why can't you just build your REST API on the Express backend?
And also, React is complex, hardly anyone seems to know the internals. The API interface might be easy to learn, but "simple" is not something that you should label React with.
I'm not sure I understand why people build websites like that. My theory would be that usually at some point you have to go lower in the stack (learn how Linux/Node/V8 work or stuff like that). You can avoid going lower in the stack, but the complexity has to go somewhere, and that's what we see here.
And also, React is complex, hardly anyone seems to know the internals. The API interface might be easy to learn, but "simple" is not something that you should label React with.