I struggle to get my head round it, too, @Babdoc and I grew up in an outer London borough, nowhere rural. When I was still in primary school, we played in the street, in a park, I walked on my own for 20 minutes, including crossing 2 main roads (and the park), to my friend's house.
Sometimes, with another friend, we used to get a bus to the very edge of the borough to play on common land and in woods or to a large park where we'd paddle in a stream on warm days. Some of the things we used to do were worrying, in hindsight, like playing on building sites and in derelict houses.
And kids still do play out, unsupervised, where I live. There 2 families with young kids in my road, all still in primary. They skateboard and roller skate up and down the road, and are allowed to go and play in the nearest field, which is accessed from the next road. I think all this builds resilience and independence
My niece, now 21, was never allowed these freedoms. She was driven everywhere as a child. Now, she's scared to get a train on her own and if her parents go out for the evening and her boyfriend isn't there, she rings them at least a couple of times to ask something daft. Her mum is positive that she does it because she's anxious alone.