Because what could possibly go wrong hacking a hunk of metal that transports you at lethal speeds...
Technical constraints to lock out owners/users only serve to enforce a manufacturer’s feudalistic rent seeking and revenue extraction policies.
In the overall scope of driving a personal automobile cellphone use or even adjusting the car entertainment system are far more common causes of accidents and death and yet these are not locked away via technical controls despite being very easy to accomplish.
Regardless, your point highlights the eternal fight between liberty and "safety and security." A society which tolerates little risk seeks to minimize liberty and maximize safety and security. A society which tolerates much risk seeks to maximize liberty and minimize safety and security.
To use cars as the prime example: why am I required to purchase a vehicle equipped with airbags [1]? They're expensive, bulky, decrease fuel economy, and may cause harm to a passenger if it improperly inflates. By owning a vehicle, I assume sole responsibility for the correct and proper operation of it as well as the physical well-being of any passengers. Frankly, I should be able to purchase a vehicle equipped without airbags to decrease the overall purchase price, thereby allowing me to use that money on other things.
The same argument doesn't apply to certain things like ABS, which I argue should be regulated because poor braking affects people both inside and outside the car. Airbags, on the other hand, only apply to people inside the car. By carrying any passenger in the vehicle, the owner assumes legal responsibility for their safe carriage.
[1] https://www.history.com/this-day-in-history/federal-legislat...
The same reason that you can't sell food that's sweetened with lead. You can add lead to your own food if you want, just like you can drive a car without airbags.
You're also right that I can drive a car without airbags, but the point is that you can't buy a new car without airbags.
Beyond that, if we assume airbags, on average, decrease injuries and deaths, then society also has an interest in helping to ensure that. Heavily injured and dead people put more of a strain on our health care system, and the costs for that are not solely borne by the person who gets injured or dies.
Certainly there's room for disagreement on whether or not all that is worth the added per-individual cost, the regulatory cost, etc. But let's remember that there are many things that, on first glance, seem to only affect an individual, but actually ripple out and affect others as well.
To put it in your terms: I'm happy for you to be allowed to drive a rat rod (hello, fellow old? I haven't heard that term in a looong time), but clearly a car with likely no crumple zones, air bags, antilock brakes, etc., etc. is less safe than a modern sedan.