Headphones only go so far. If you don't have good headphones (or, gasp can't afford good headphones, or don't have the time or money to shop around for good headphones), then you end up with a band-aid that doesn't really work because... well noise bleed is real. You either still hear what's happening around you or the people around you can hear your music. So they talk louder, because they need to make sure they're heard by whoever they're talking to. They don't stop the distraction of someone walking by. They don't stop someone from trying to start a whimsical conversation just because they walked by you.