I'll just guess, but I'd say it's because the top two things, besides cost, they're looking for is: 1) that it doesn't unbuckle under any survivable load and 2) that when it goes "click" it's always locked. Obviously a latch that released on impact would be bad, and a false lock would be bad, too. So they're probably designed to latch harder as they're loaded.

I would bet that the design of airplane seatbelts is different, that they're designed to always be easy to release, at the expense of not being as certain to lock, because the risks are different.

Or it could just be dumb luck and bad engineering...