IDK how it does it, but the automatic seat belt locks engage really quickly in my F250 based on changes in acceleration in the vehicle (which was super annoying when I had a constant vibration caused by sh1tty tires). To the point where I had a hard time reaching anything
. I'll agree that in most cases they're slack, so your point is valid, though there is always some form of tension on the 3-point belts to keep them taught and against your body so you can't slip out of them easily or by accident.