People don't fasten seatbelts until the chase scene is well under way.

Before fastening them, the characters had somehow managed to retain their seat positions while driving on extremely bumpy roads, skidding around corners, etc., so there seems to be little need to ever actually fasten them.

Fastening seat belts is a signal to the audience that the action and special effects are about to ramp up.

Cars explode when shot at.

Most fuel tanks won't explode unless they are nearly empty. Even then, bullets are unlikely to cause sparks to ignite the fumes.

Exploding cars and balls of fire are incredible special effects. If you can make a car explode, why wouldn't you?

Even modern computer-controlled cars can easily be stolen by hot-wiring them.

Hot wiring requires knowing the purpose of each wire. It requires connecting several pairs of wires (corresponding to the clicks when turning an ignition key). Only after everything is connected properly can the sparking wire thing be done to briefly supply power to the starter motor.

Modern cars have RFID detectors and internal locks, and many don't even use keys anymore. And it's all computer controlled, so even if it could be done, the appropriate wires are no longer located in the steering column.

People falling out of windows always land on top of a car, setting off its alarm.

Most windows don't have cars carefully positioned below them.

It's become such a cliché that it would feel wrong for a falling body not to land on a car roof.