It confuses me even more considering that Google is behind it. Google just announced fonts for all unicode characters. [1] Why create an extra font that will cause every app to look like a "Go" app instead of a native Windows/Linux/Mac app?
But as keeps getting said: it's about the licensing.
I still don't understand why they would have to license the Google fonts for testing and using them as standard fonts. EDIT: I just saw that Noto is available with an open license, just not the Go license. But I still think Google would relicense it to Go, seems like something Google would do in this case.