Comes down to how you are most likely to die.
You are very likely to survive the plane crash. You are less likely to survive the ensuing fire, and will probably die of smoke inhalation.
A complicated seatbelt is the last thing you need as your skin melts and your lungs fill with carbon monoxide.
With car seatbelts, they are designed to keep your head from smashing through the steering wheel and into the dashboard. That kind of thing is pretty fatal most of the time. Cars usually don’t catch fire and explode after a crash. Which is why seatbelts in cars are designed to save you from the collision most likely to kill you (the one with the dashboard).