You can absolutely fuck with your car and install after market parts. But your car still needs to pass a minimum level of road safety. That’s for the common good. I mean things like having correct lights, crumple zones, brakes, mirrors etc. Now if you want to make a car that runs on used kit hen scraps I totally think you should. We should own. Own devices including their computer system. But I don’t just the average Joe with the parts that keep me safe from them.
Also, this might not shock you, but cars also used to suck in many ways. Not everything from the past is good.
I think sooner or later open source cars will be a thing. The problem (well, it’s a good thing) is that cars need to pass inspection to be road ready. But not bike-like vehicles.