Google Docs did this for a specific reason. Browsers restrict access to the clipboard on general user actions (like click, right click, etc). On the other hand, if you use ctrl+C or ctrl+V, the website gets access to the clipboard, because it's a JS clipboard event. So those work in Docs. Think about it - do you want any website to just read everything off your clipboard?

Docs overrides right click because there's things you'd naturally want to do in a word processor with right click (format, link, comment, etc). Because docs shows a custom menu on right click, the browser will not expose the default menu, and without the default menu, there's no JS copy/paste event.

Browser extensions have more access, and that's why a browser extension re-enables the functionality.

So it was a design choice: the Docs team decided that giving users all these other right click actions was more important than allowing the user to right click to copy/paste without an extension (since most people use the keyboard shortcuts anyway).