Hipster value. All the cool kids are now using Github.
You know, the point of open-source being that everyone should be able to contribute and all that.
Microsoft has caught on to that, hence the move.
Lets for arguments sake accept your statement as fact. If that is a fact, is that inherently a bad thing?
If I use a piece of software, open-source or not, will I or will I not benefit from that software somehow obtaining better test-coverage? Ofcourse I will.
If that software is important to me, being able to contribute with tests or other "boring" or "light" material means that the core team can focus on further development. And I honestly don't have a problem with that.