21 years ago, my daughter came home from hospital, unstrapped, in a carry cot - also unstrapped, in the backseat of my father's car. This was legal, by the way, at the time. Was I happy with it? No. But it was legally okay, and - to be fair - my father did drive very slowly/carefully that day.
When she was 8 months old, we went out with her godparents for a day - and at the end of it, her godfather had hold of her/said he'd put her in the carseat. He only strapped her bottom half in, had no idea that he needed to put her arms in the harness, too. I only discovered this on a dual carriageway, where I may have yelled a lot about baby safety (actually, there's no "may" about it: I did).
My ex is renowned for not strapping our son/his other, younger, children in to their carseats. To the point where, one morning I watched his mother drive my son/my ex's eldest daughter off, came back into the house and called the NSPCC to report them for the fact that whilst my 9 year old had been strapped in (because I made him put a seatbelt on), his 5 year old sister... wasn't. They had a visit, I'm told, and seatbelt safety was explained to them because, since then, the little ones have been safely secured. And no, I couldn't tell ex's mother about the danger because I've tried. Every time I check my son's seatbelt, she starts on about how her children never needed seatbelts.
And no, my generation didn't have to wear seatbelts. But that didn't mean that we were safe in the event of an accident at all. The reason as to why the law changed was that scientists worked out how dangerous not being strapped in a heavy, quick-moving vehicle actually is. Especially if you're very small/young.
My son has had it drummed into him so many times about seatbelts, though, that he was reaching for them as a 2 year old to make sure he was strapped in. I can't control what my ex does with the children he had with his wife... but I can/do still worry about their safety.