Why would it (legally) be on the car manufacturer if someone hacks his own car and causes an accident because of modifications to the ECU (firmware)? This does

Why would it (legally) be on the car manufacturer if someone hacks his own car and causes an accident because of modifications to the ECU (firmware)? This doesn't intuitively make sense to me. At the very least there are probably huge differences between countries when it comes to this? Aside from the fact that some people would likely love to modify their car in every way possible to use it on the racetrack or whatever private property?