I would have thought that "time to lock" would be the problem here. Give a car seat belt a good hard yank, and see how much belt pays out from the reel before the reel locks.
With the current hard-fastened buckle system, seat belts stretch by up to 10% of their length in restraining the pax - one of two good reasons to make sure that when your belt is fastened for take off and landing, it is done up tight.
If you add to that much inherent "slack" in an emergency system the additional amount that would pay out from an inertia reel, you would be seriously eroding the safety margin.