Well you don't have to be born here either. The difference between a visitor and a resident is that visitors are more like tourists. They don't feel that pull, they aren't wanted in the town and after the Horror feels like they've seen enough, it'll start psychologically pushing them away, making DH seem unappealing to the person and annoying them until they leave.
Residents don't even have to have arrived by the start of the story (though they should be arriving imminently, since we only really RP in the town). Residents are lured in by the Horror. They want to live there forever. It's the opposite of a visitor, who arrives on their own volition and leaves after being shooed by the Horror. Most residents don't even get why they did it but they showed up anyway. Most can't leave without looping back to the town thanks to the fog. But some do get to leave, like spores from a mushroom, to seek out new victims and lure them in too.
Edit: okay, I've asked a bunch of other people too about the difference. I think I get where the confusion lies, so I'll put it as plain as I can.
Residents = characters who intend to (or presently) live in the town. They're the ones allowed to stay. In fact, they're not even allowed to leave. You can roleplay your character at any point in the moving process but they will eventually end up in DH to stay.
Visitors = characters who visit the town of their own volition. Outsiders. Typically NPCs. Cannot stay in the town for more than a couple weeks. Unable to get involved in the larger plot nor take part in encounters, because the Horror makes them leave before they discover anything interesting.
Delville Heights maintains its secrecy that way. If everyone was able to come and go as they please, I'd have given it two months before the government quarantined the area.