Usually when a car is in an accident the rescuers will just cut the belt up high where they can easily see/reach in, espically if the door can't be opened.

Also seatbelts are designed to stretch some to absorb the impact and the pressure on the latch can cause it to jam. I've read that it was a tradeoff in design, to make the metal buckle stronger to where it doesn't deform some in an accident it would need to be a lot thicker and heavier and harder to latch/unlatch so it would discourage use.