As far as I know this is due to the fact the driver is using some sort of controls on the car. Only 1 player can exit the car at the same time, and the same rule applies to any vehicular control. If the driver was holding the gas or brakes when they disconnected the car will not allow passengers to exit until the driver has fully disconnected and been kicked from the server. (Could be wrong about the gas or brakes but certain actions in cars block any other player from being able to exit the car or close doors while that action is currently attempting to or being preformed)

In T183026#2641204, @Osdaeus wrote:

As far as I know this is due to the fact the driver is using some sort of controls on the car. Only 1 player can exit the car at the same time, and the same rule applies to any vehicular control. If the driver was holding the gas or brakes when they disconnected the car will not allow passengers to exit until the driver has fully disconnected and been kicked from the server. (Could be wrong about the gas or brakes but certain actions in cars block any other player from being able to exit the car or close doors while that action is currently attempting to or being preformed)

You can - in fact, as a passenger, leave a moving car. I've personally done it. However unfortunately a person desyncing completely breaks the mechanics until the driver disappears from the game. Now imagine you are getting shot at, driver desyncs and you sit there, unable to leave the car for 15 seconds. Would suck.