The question is: why do airlines encourage PAX to bring as much crap as possible into the cabin.
From my personal experience PAX are deterred from putting luggage into the hold by the costs levied by airlines, also the wait times at their destination, the risk (albeit small) of luggage going astray or it missing a connection etc.
Better safety instructions are likely important regardless, but i.m.o. only a lock would make everybody consciously aware of the issue and actually adapt behavior and packing habits
Absolutely right. If you knew the overhead lockers to be were locked whenever the seat belt sign was lit, in the event of an emergency or whatever, you'd make sure you had the essentials (passport, phone, wallet, meds, phone chargers...) on your person. Sheesh before you know it the airport shops would be offering all manner of shoulder holsters for this stuff.