If people NEED someone, because they’re lost or in trouble or there’s an emergency, they can ask for my help whether I’m wearing headphones or not, and that’s fine. Obviously.
If people WANT to talk to me because they just want some attention or they’re bored, then that isn’t my problem. Their preference to have a conversation doesn’t trump my preference to focus on listening to an audiobook. I don’t exist to provide strangers with a way to pass the time.
People are entitled to keep themselves to themselves if they want to. If I’ve spent all day at work, talking to people and communicating (my entire job is basically words and using them to engage and persuade) I want to decompress and be quiet in my downtime, thanks.
Also, a lot of people wear headphones because they need to focus on something to relax. Some people relax by letting their thoughts wander. Some of us just find that stressful - for some people, wandering thoughts always turn to worries or anxieties. Or people might be out and about doing errands or exercising or commuting and might just want to listen to something to make it less fucking boring. Last week I had to spend my lunch break dropping off Vinted parcels at three different locations; I don’t drive so it was an hour of walking and would have been a really annoying chore. But I put my earbuds in and listened to an engrossing audiobook and it made it into something pretty pleasant. An hour’s chore became an hour’s entertainment.
If I’m on a train journey, I read a book. If I’m walking, I listen to one. Sorry if that means I’m not giving the impression that I’m available to be the world’s guardian angel.