That would essentially be a merge with BitBucket, so people would complain about there being less choice.

Atlassian would have pretty much no reason to buy GitHub, unless it's to shut it down.

They have Bitbucket and I think is pretty safe to say that they don't care about people hosting their open source stuff.

Of course you can use Bitbucket for free but where the big bucks are for Atlassian is the enterprise market. I don't have market data but I don't think there are many big corps using GitHub as opposite to Bitbucket together with other Atlassian products.

You'd be surprised. Case in point is my current main client, a huge national broadcasting company which is using Jira and Confluence for everything on one hand, but is just setting up a GitHub private instance on the other.

I'm indeed surprised and I'm sure there are other cases like this out there, it would be interesting to know the thinking behind it.

In my experience of working with big corps, if you are using Jira and Confluence, Bitbucket simply offers much better integration with them and lately Atlassian is rocking it with new features and deeper integration every time.

You are absolutely right from the technical perspective, but I guess these decisions are based on buzzword matching rather than any sound technical design.