> I'm curious of what is missing
My initial response was related to missing comments and collaborative real-time editing in the v1 link you provided. However, I searched a bit more and found that their newer API versions include both of those features.[1]
Here's a blog that's announcing their real-time collaborative editing[2] back in 2013, so that's been there for a while.
That blog post also lists a few products that make use of the real-time feature but the only client that's an actual editor ("Neutron Drive") has since died with its domains no longer working.
I guess this is one of the rare cases where you can build a client based on their API and Google actually promoted one of them but I'd still say that it's not really meant for it.
The primary use case for these APIs are bot services like Zapier and integrating a "Save to cloud folder" feature in your own app.
Even if they actively encouraged you to build a client based on their API, who would risk it? We've seen how Twitter handled 3rd party clients after it got big and that's likely going to be the case with every major player.
[1] https://developers.google.com/drive/api/v3/manage-comments
[2] https://developers.googleblog.com/2013/03/build-collaborativ...