Haha, I'd think only people who were never in a car as a child wouldn't know that. An adult might not mess with them enough to suss out that behavior, but kids loooove playing with seat belts.
My brother-in-law is large enough that when he pulls his seatbelt out far enough to actually go around him, it locks, and he absolutely hates it.
Is there a reason this "feature" even exists?
Edit: Gah! As a Dad myself I am compelled to add that his lack of experience with carseats is clearly aparent to me.
https://csftl.org/whats-the-deal-with-weight-limits/
Personally, I don't think that was a good threshold to set. I agree that the LATCH anchors are better. In 2003, I suppose people were mostly using carseats for infants and toddlers in little 10-pound styrofoam shells, now we know kids are safer in carseats until they're significantly older and heavier, and they're in 25-pound adjustable, heavily bolstered bucket seat style carseats.
Really, the list of hidden features on products is such that nobody has solved discovery. Affordances that work are ones that mimic already learned behaviors. But shared learning is not as universal as folks think it is.
Interactive systems that let you ask "what will this do?" Or "why did that happen?" Are very good. But even that is hard to work with sometimes. Consider how few folks use the apropos utility.