In fairness, there is almost never one bit of explanation of *why* people are asked to do these things. I fly in the developing world a lot and no one from places like China has decades of education about the dangers of anything, not that the seat belt part is that much better anywhere else. Cabin crew don't set the agenda for what's discussed in the safety briefing, I know, it's just that the bureaucracy has forgotten the human factor. Mentioning why people need to buckle up would go a long way to helping enforcement. I end up having to explain to people to buckle their kid up because I don't want him breaking my neck, or his, if he flies out of his seat - and often they're actually pretty thankful that someone explained it.
There is zero, zero education about this stuff for the general public who don't actively seek out information or watch air crash / air disaster content, basically, so they just think it's like the train or the bus or a taxi.
"This one has a z-axis in all situations. Fasten the Goddamn belt!"