There's more to it than that. MCP adds the concept of the API endpoints describing themselves (giving agents context), standardizes how auth works, and so on. This could all be done by packaging existing standards, but MCP in many ways is packaging and organizing existing standards, minus the previous stateful-first design.

I do think a lot of the standard would have been easier if they'd been stateless first, and allowed for GET requests, but it evolved from a starting point that has since evolved.