When a designer is working in photoshop they are not actually part of the development process yet. They are still in design mode, they are for the lack of a better word sketching.
Not until they start to actually output assets for developers do the github concept start to make sense.
Designers often have different versions of the same design inside the document in the layers and groups and smart objects.
They will for instance have a couple of version of a header or some styling on their elements, perhaps different layout for main content.
So for this to be useful it would need to create a kind of master psd that save the state of which layers and groups are turned off and on.
This way you can sketch away and not have to worry about creating a billion different versions inside the documents that just make them bigger.
But that is a different problem.