They did...

Google Drive Release: http://googleblog.blogspot.com/2012/04/introducing-google-dr...

Google Drive SDK release: http://googledevelopers.blogspot.com/2012/04/introducing-goo...

Actually it was released a few months ago. The Google Apps Challenge 2012 [1] which started on June 5, is about apps built on top of it.

[1] http://www.google.com/events/gadc2012/

For the most part you could piggyback off the Google Docs API and SDKs if need be.

It likely didn't exist yet. These things take time.