I actually think we're seeing a bit of an awareness gap. Car seats started to be required/standard use around the late 80s/early 90s. So anyone older than this (born 70s/earlier 80s, children born during 00s) won't have had car seats as a standard thing and will have looked up the rules or sought advice when their DC were born and probably stuck to them as a totally new thing.
In addition, for this cohort you had the rule change in 2003, from car seats being required up to age 3, to the new law that made booster seats required up to age 12/135cm. There was lots of publicity about this, because it was seen as a controversial change with many people having stopped using seats and so their DC would effectively have to go back to using a booster seat. So most people with children this age were well aware of that change and used car seats up to the required age. Group 123 seats (forward facing converting from harness to booster seat, lasting 9m - 12y) became hugely marketable and popular as an alternative to the standard Group 1 forward facing, 6/9m-4y seat which had been the previous most common option, available since the 1970s.
The parents of primary aged children today (born 2010s) were likely born in the 80s/90s themselves. They probably used car seats as children or saw them being used for younger children, but throughout the 90s hardly anybody continued with booster seats after about age 4/5, despite awareness campaigns - this is why the new law was brought in. At that time this cohort was in their late teens/early 20s, probably didn't have much contact with children under 12, probably paid no attention to news reports about car seats - not relevant to me/not interesting.
So you have 80s/90s parents and 2010s children. These parents expect to use car seats for younger children and probably won't have specifically looked up the law, they will have simply gone into a shop or purchased online a car seat that says it covers children from birth/up to X age. In addition by the time these children are growing out of their big/toddler car seats, it's been about 15 years since the 2003 rule change so there's no publicity about it any more, it's taken for granted that everyone knows. Because booster seats are more widely used they are now for sale everywhere so they typically do use one, but expect to stop using it around 5/6 years, unless they have happened to look up the law. Also, 2016+ the spinning type toddler seats have become far more popular and the ones sold at this time tend not to have a booster conversion.
So these DC are currently towards the later end of primary and what you're seeing OP is not unusual. Many of these parents are totally unaware they are breaking the law. They just assume - common sense you need a car seat for young children, older children don't need one, only overprotective people continue to use it.
I think this will self correct over the next 10-15 years, because once you get late 90s/early 00s born parents having children, they will have the experience of themselves using a booster seat until late primary school, and so are likely to repeat this with their children.
And of course individuals within any generation will have their own unique approach, what I'm talking about is general average trends - e.g. if you surveyed people, on average, without being allowed to look it up I think Millennials would give a lower age for the legally required car seat use compared with Gen X (who were likely to be more aware of the law change) or Gen Z (who likely experienced the new law themselves as children).