>That's just the thing. Why has there been so little adoption of commonly needed functionality as part of core, or at least as officially supported plugins?

Oh there are core bundled plugins, official etc - they are just core functionality that happens to be implemented by plugins.

>And why is version control not standard in 2018?

that is and always has been - "git" support used to be not in by default, but that was a while ago (it is included now).