The site only allows .STL and .OBJ files to be uploaded, even if they are inside a zip file.
This means that it becomes very hard to create derivative works because you cannot upload source files such as:
.prt, .sldprt, .asm, .sldasm (Solidworks)
.dwg, .dxf (AutoCAD)
.stp, .step (General)
.ipt, .iam, .ipj (Inventor)
Users have to reinvent the wheel from scratch.
Also take a look at some thingiverse posts and there are no .stl files, just .scad files so the users can modify the design to suit them. (Search "scad" on thingiverse and you get over 3900 results)