For any other kind of machine that can kill people, you are solely responsible for any accidents that could have been caused by your modification. I don't think that it's so unreasonable to want to apply this to cars too.
That's how you get wheels falling off from not being torqued, bearing failing prematurely, brakes improperly adjusted or pads popping out, and the list goes on.
Unless you have a tech you know and trust (preferably an independent garage) do not rely on the fact they are licensed to guarantee safety. I could spend the next hour listing off instances of shoddy or incomplete work by licensed folks.
Almost all of this as applied to hobbyists is corruption.
The amount of danger you can put yourself or others in without any involvement of automobiles is immense.
If you build an electrical device improperly it may catch fire and burn down your house and your neighbor's house and spread from there.
If you mix the wrong chemicals together you can cause an explosion, or produce poison gas, or create a large volume of something difficult to safely dispose of. These chemicals include things like propane and household cleaners that are available to anyone.
If you maintain unsanitary conditions where you live you can contract diseases that are communicable to others.
What prevents this from being more common is by and large not regulations and inspections of private citizens. It's the fact that people don't want uncontrolled fires or poison gas or staphylococcus and are generally cautious when doing things that are potentially dangerous.
And people don't want to destroy their $30,000 car or kill anyone either.
That's the point.