I grew up in the 1970s in a village. There were fewer cars on the road. There were also more adults about - not just mums, but unemployment rates were high in our area, so more adults at home all day.
To be brutely honest neither of these were the reason why me and many of the children in my street were allowed out all day unsupervised. It was because, to be frank, it would never cross our parents minds to keep us in. They were not going to bother 'entertaining' us as they had never been 'entertained' themselves as children. My parents must have been fully aware that in the summer were playing in the local river, or walking miles from home with no way of contacting them in an emergency, but they just seemed to accept that there was a remote possibility that something bad might happen, but it was just less work for them if we went off on our own anyway.
I remember the odd reminder about not getting into strangers cars but nothing more than that. I think the assumption was that we were all out together and that there was safety in numbers.
If we had stayed home what would we have done? TV was limited to three channels with very little children's programming. We had toys, but not that many as money was tight. Swimming lessons cost money so that was out - and we didn't have a car so could not have got there anyway.
I did have friends who had parents who were more involved. They could only play in the garden and were signed up for brownies, Sunday school and swimming lessons. They had at least one working parent at home. I do think my parents neglected us as both had nothing else to do all day. But they were just repeating the parenting they had experienced.
My children did not have those freedoms - and lost a little in the process. I think they are less resilient and less independent, but that was a small price to pay. I had the money to sign them up for activities, and of course they had far more 'stuff' to keep them at home - more TV, more gaming.