Don't care about their own safety
I think there are several things at work here. One of them would be that probably 95% of the pax have not the slightest idea what "severe turbulace" means in terms of things happening in the cabin. If they did have any idea, they would probably consider it the captain's fault for letting it happen (or maybe for 'causing it').
Most people probably think 'severe turbulance' means they have to hold their drink cups because they will bounce off the table. Or maybe they will bump into the person in the next seat. Very few probably ever think that it might mean flying across the cabin and landing on someone on the other side 5 rows back.
Maybe the safety films should be more graphic. Show a cabin of flight test dummies and shake it up so that the ones not strapped down smash into seats 20 seats away from where they were seated. Show lots of people piling into a pile against one wall which is now "down", then being thrown in mass to the other wall. Then tell them that will 'probably' happen to them the next time the seatbelt light comes on if they aren't bolted down.
As for cabin crew walking about. In many industries if you work on a platform where you might fall, you need a safety harness. This usually goes to a slider that runs along a pipe above you where you move. If you are knocked off your feet, you don't go very far. Maybe there should be safety harnesses for the crew and slide pipes along the top of each asile.