that is because they are keeping themselves open to have a Steam packaging format.
It is something I have always bet that they will eventually do - once they see the number of support questions by people who have some sort of dependency problems (e.g [1], [2] and [3]).
I wonder how long will it take for them to latch on to something like Zeroinstall or nix.
No, I understand not distributing the games in DEB form, but what's the point of versioning an installer that then goes and downloads the latest version of the client? Just put the client in the repo...