Because the modification of that computer is order of magnitudes more difficult than a mechanical modification of a moving part. The humanity / regular human is able to understand much better the interactions of the mechanical parts that usually are always local and well defined.

This does not exist in the Sw, FW and digital hw, the interactions are not local and are millions timesmore complex to understand and properly modify.

It would be an utterly irresponsability to modify (blindly) the Sw of an xray machine that could make it unsafe and ultimately it could kill humans, and it is the same concept with the car. It is irresponsable to make a modification that can make the system unsafe.

For the rest? Regulations, free software foundation and good selfhosting Cheers