I don't understand people who say things like this
Surely you have met people who talk loudly before?
When my neighbour sits in her garden and I'm in mine, there's approximately 2 feet between us, albeit part of that blocked by a fence. We're closer than I am to my own backdoor. I can hear her better than my family in my own house, so when she is talking full volume about her relationship it is impossible NOT to hear her, as much as I'd prefer silence.
Unless you expect people to wander round their OWN GARDENS with headphones in constantly, and thereby risk not hearing their own family call to them, or the doorbell going (and also, just, why should you!), it's not 'obviously listening in' out of any sort of nosiness, it's being physically incapable of NOT hearing.
OP isn't saying she does anything with the information she overhears, just that she can't help hearing it and wonders why people are happy for their neighbours to overhear what can often be quite private and confidential conversations.
Which makes perfect sense to me, my neighbour has never stopped me in the street to tell me how well her ex used to fuck her, so I'm assuming it's not information she particularly wants me to know, therefore OP querying why people seem to someone think a small fence or hedge magically renders them inaudible is a fair point.