The difference is the game developers have put in extra work in order to make their games easily moddable. Your car didn't. Developers don't have to add mod support, but then it usually becomes incredibly difficult to modify.
They put extra effort in to allow people to create stuff using their game. It's like using any other software as a tool; you need to follow their rules for using it. This could mean paying a licensing fee, or it could mean making it open source, or whatever else they may decide. You're using their tools, so their rules have to be followed.