There is no such thing as "ALWAYS" unless it is death and taxes.
What, exactly, are you referring to?
Weather can change quickly in unpredictable ways from better to worse, no matter how sophisticated the technology and predictions.
Whenever in a plane, usually as a pax sound asleep, I keep my seatbelt fastened unless I have to get up and move around.
If pax refuse to heed the "fasten seat belts" signs and announcements, do you expect cabin crew to pass by every ten seconds making sure everyone stays buckled up after several warnings?