35 plus years ago, I was in a TAA DC9 or 727 flying MEL - HBA.
We hit SEVERE turbulence between Launceston and Hobart and I will NEVER forget the look of terror on the face of the flight attendant as she was spreadeagled on the ceiling of the aircraft above me!!! She was looking down at me, and I was looking up at her.
It was a meal service, and there was no announcement of any turbulence (before radar could see it).
I also remember my arms being totally weightless while this was going on. And yes, I was a good boy and did have my seatbelt on.
I also still vividly remember the flight attendant coming down from the ceiling and being spread over my seat and the one next to me.
She was screaming and crying, half the rest of the plane was doing the same, the meals were everywhere, including all over me and the plane was a total mess.
We got to Hobart and were met by a number of ambulances and medial staff.
The girl who came down on top of me had either a broken arm or broken collar bone - I don't know which, but she was quite a mess.
It made the local news and cleaning the aircraft must have been a real challenge.
I will NEVER not wear a seatbelt when I am in an aircraft, other than the usual comfort trips to the loo.
The problem is that about 99.9% of passengers, and even a large percentage of the crew, have never seen severe turbulence. So, the upshot is that the level that is about mild is assigned as severe in most peoples' minds. And whilst that might make it difficult to walk, it really isn't a patch on the effect of the real stuff. But, as people think they have a handle on what it will look/feel like, they decide that seat belts are optional.
A flick to the roof at between -.5 and -1 g would be bad enough, but then you come back down at 2 to 3 g. And you won't land in your seat, but on someone else near you. Or the armrest. Or something else inconvenient.