I've always thought it had to do with control - in a car, you "have control" over your vehicle, whereas in a plane, you don't. Which is essentially the theory you're referencing.
If it was simply a control issue then that wouldn't that imply that people should be as scared to take the bus or the train as they are flying in a plane?
I don't doubt that there's a component of control involved, but I expect that the whole "we're flying in the sky and if anything goes wrong we're all dead" has a bigger part to play where fear is concerned.
