The difference is the number of engineering disciplines involved. A smartphone, as complex as it is, still requires a relatively narrow set of disciplines compared to an automobile.
Systems engineering was basically created by the auto and aerospace industries for this reason. It coordinates all these disparate disciplines into a single, cohesive effort. What I have seen of systems engineering[1] in SV does not leave me with the confidence that most of those companies will actually succeed beyond some science experiments. At least, not the companies that haven't started out as fundamentally car companies like Tesla did.
[1] Real systems engineering, not glorified, overworked sysadmins