How does the existence of Google Docs and Word say anything about how hard it is to create real-time collaboration on a web application versus a native application?

Not really. It all comes down to a network protocol to sync changes. I collaborated with others just fine in desktop-first apps like CounterStrike or Diablo long before web became a viable platform.

The reason MS Office struggles with this is that rebuilding an app that wasn't originally supposed to work in such mode is way harder than building from scratch, and they probably don't really need it that much to push themselves in such an adventure.

There’s no technical reason why that would be true. What is true is if you have an app and doc format that weren’t designed for real-time collaboration, it’ll be really hard to retrofit, regardless of your choice of app platform.