Making a good mechanical design is difficult, it usually involves making several iterations which in the physical world takes a lot more effort than iterating on software.

One thing that always bothered me is that most people with 3d printers seem to design things on their own from scratch and rarely take on others designs, improve them and share them. There is little collaboration going on for 3d prints in comparison to software. Except from maybe ~10 widely successful projects that now have healthy communities improving them.

Why is GitHub and similar sites not used more among makers?

In your first sentence, you answered your question.