1) I doubt you're correct about the complexity.
2) The EULA is at largely about "we take no responsibility for whatever breaks." This would never fly legally for cars because failures kill people. Also, it's less reasonable; since you're unlikely to install arbitrary programs in your car and open it up to the internet for attack, whatever goes wrong likely IS the maker's fault.
There are any number of modifications people can make to cars that make them more dangerous through no fault to the maker. On some cars, not making modifications is something of an exception, think of Wranglers.
The difference I think is that if someone modifies their jeep and ends up killing themself, it is rather easy for Jeep to prove their innocence; no matter how wrecked the car is the modified suspension or whatever will still be rather apparent. Imagine the nightmare that would be for a software company though.
Of course proving innocence is not how things are supposed to work, but...
Not that I'm a fan of EULAs.