system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are airplane seat buckles different from car buckles?

Basically the title. 

I’m guessing seatbelts have the same purpose on both, but one goes only on your waist (plane) while the other is on the chest as well as the waist (car). 

Is it a cost thing or a safety thing or something else entirely?
gpt
The seatbelt in the car is trying to dissipate the energy of the crash.  If you're traveling down a highway and hit a wall that brings your car to an abrupt halt then your car may experience 100 Gs of deceleration.  If you were firmly attached to your car this would be bad for you as 100 Gs will break your neck.  The car seatbelt is designed to stretch and give your head/torso that half meter between the headrest and the dash to decelerate over to reduce the force applied to you in the crash.  It needs to run across your chest to control where your head goes and to let it dump energy into your strong ribcage

The plane seatbelt is to keep you roughly located in the seat.  There are no high acceleration events in a plane that you can survive, at most its trying to deal with extreme turbulence which might try to throw you a couple feet either way.  Its there to keep you from flying during turbulence and getting hurt or hurting other people.  There isn't much middle ground for a plane crash, either it skids down the runway on its belly in which case keeping you roughly located was good enough, or it hits something going way too fast and there's nothing a better seatbelt could do to help you.  Because a better seatbelt won't help you want it to be as simple and unobtrusive as possible so its used more often and helps when it can.