It's a very astute and appropriate question & one I touched on in a blog (link at end of twitter rant :-)
There are a few reasons for githubbing. The ones here are far from exhaustive.
First, many folks are fleshing out an idea (so it's a short-or-long WIP) and also looking for collaborators and testers. This is a good/necessary iteration step IMO as it garners attention from the community and one can get a sense for the utility/desirability/reach.
The "CRAN" hurdle is a serious effort that all pkg authors (even if they aren't on GH or CRAN) should strive to vault since the checks associated with it really help with overall package quality.
Now, I'm as "guilty" as anyone else when it comes to having GH-only packages and I do it for many reasons.