Why offset the rear rim to the hub and not the hub in the frame?
The biggest functional reason actually would be chain line. 

You could make the hub shell wider, and run an offset dropout to allow the space for the gears, but then you would have to run a similar offset on the bottom bracket to maintain a usable chain line.

Running the offset on the bottom bracket would affect Q factor positioning on many riders, and while some riders like or need additional stance width, most do not.

Because a change like this requires such a radical redesign of pretty much every bicycle component, it would require a consensus from the entire range of bike manufacturers to be put into practical or wide spread use.

So you could call it tradition, but I think momentum would be more accurate. The existing body of design work would need to radically change to accommodate something like this, and that is a lot of "mass" to shift. 

Since existing wheel structures and designs are working for riders as is, most companies choose to stay within the range of difference that allows the bike frame and other component designs to be affected as little as possible.

After all, there is no guarantee that an offset design would work in practice, and the benefit to it isn't major, given that existing designs work.