I wonder if the OP's intention was to give people who have never owned dogs a place to vent their spleen? It feels a little like that.
In 2023 research was carried out that indicated that 31% of the UK households owned one or more dogs. In 2021/22 that worked out 13 million dogs in the UK. If all those dog owners failed to collect their dog's poop we'd have a lot bigger problem than being misrepresented here. The vast majority do pick up after their dogs.
Where I used to live the only problem owners we had were a couple of husky owners who couldn't control their dogs and the locals convinced them to walk their dogs elsewhere. The second was a jogger who looked like he had money judging by the expensive kit he wore. He would let his two labs run free and was too busy running to care about his dogs. That canal towpath was often fouled by a big pile of lab poop! That was until a local character who lived in a canal boat came back to the area. A dog owner himself he took it upon himself to police the towpath dog owners who were irresponsible. He was a bit of a psycho too who lost his temper at times, Needless to say that runner picked his dog's poop up after he was confronted. As that psycho guy told us the irresponsible dog owner told him he had no poo bags, but the psycho still got him to pick it up by hand! He learnt a lesson that day!
Seriously though, IME there's a minority that cause most of the issues. The other point is you are more likely to see those poo bags left in countryside areas by people visiting the area from outside, townies taking their dogs for a walk in the countryside. Whatever I've lived, in rural or semi rural areas, the locals tended to keep their patch relatively clean. Within those areas the spots outsiders would visit to exercise themselves and their dogs there would be issues. Often full poo bags in carparks or thrown over the wall from the path, things like that.
However, I towns, urban and suburban areas no see the locals also picking up after their dogs. You do however get problem spots in all areas. The guy upthread with the grassy spot with trees used as a doggy toilet do exist but they're often used to blow the issue out of proportion.
Incidentally when I lived in a small local town there was a small, rectangular green space for communal use, often used by kids and dog owners. It was kept immaculately dog poo free by dog owners who used it. However, it was still absolutely full of cat poo. Even fox poo. Trust me I know because my dog has rolled in enough of it to stop us using it! Fortunately they mostly went in the few flower borders not on the lawn areas kids played on. Perhaps we should be making cat owners scoop their own cat's poo up because in that area it was the cats that were the source of the local poop problem. Well them and the Friday night vommitters who frequented a certain pub!