I have contact with kids from 3 different places, 2 with high independent mobility and 1 with low independent mobility, and as much I like to agree that kids needs to be free, there's an important parental argument that needs to be talked about that is risk vs reward function if the kids get hurt.
In places with high mobility (at least 2 of them in the chart) there's some state support in terms of children's sick leave if something happens, plus work protections if you need to be absent for more than 6 weeks, and the education system has mechanisms to not let this kid be left behind (for example, if a kid breaks his/her legs).
In those places with low independence, I talk with some parents, and all of them are scared of the possibility of something permanent happens or something that can demand continuous support during working time; in those cases I can see why they play safe.
In the other hand, another second-order effect is that in those places with low independency, one thing that I noticed is that the motor coordination takes way more time to develop, and it cascades down for instance during sports activities (of the lack of), physical development and so on.