> The same countries that made car part serviceability a legal requirement, also made an engineer's license or degree a requirement to design such parts.
No they didn't. Anyone can create a car part and put it on their car.
There are a small handful of exceptions where the components are supposed to be government approved (headlights and tires are two examples), but in practice this requirement is generally even more ignored than the speed limits.
If you have a suitably equipped garage, you can build the entire vehicle from piles of metal.
There is not degree or engineers license required to do this.
Now, when you start selling parts to other people, there are more requirements.