Only the obnoxiously hard headed folks would argue back and go "since it's a recommendation and not a requirement I will not sit with my seatbelt on, and there's nothing you can do about it."
Except as provided in this paragraph, each person on board an airplane operated under this part shall occupy an approved seat or berth with a separate safety belt properly secured about him or her during movement on the surface, takeoff, and landing.
https://www.ecfr.gov/current/title-14/part-121/section-121.3...
I've never looked for stats on these incidents involving clear air turbulence. The impression I got as a European was that this was a transatlantic and transpacific problem. It's obviously not, but those were the incidents reported on.
According to the below, long haul flights get it more, due to the altitude they fly at.
This of course doesn't mean one shouldn't wear it as often as possible, but ie with small kids thats nigh impossible, or for some folks on 10 hour flights (I still used to put it on me when I slept, but mainly to not be woken up by crew when they switch it on and check everybody).
Very similar topic is bus seat belts. All intercity buses (at least in Europe) have them, nobody clips in (apart from driver). I think at this point everybody knows about some horrible bus tragedy which would be a set of minor scratches if all folks had seat belts on. Yet our subconsciousness keeps changing perceived risks and consequences to keep us happy/content instead of worried about everything all the time (makes some sense, cluttered mind ain't best performing in life & death scenarios of bygone era).
