It's not uncommon with the post-apocalypse genre. Many of them want to give the sense that you're the last person alive. And given the lore for 76, it would have made sense that there wouldn't be any normal people on the surface (it takes place when the first vaults began opening after the bombs fell). Although, why not have ghoul, super mutant or robot NPCs?

I was playing it recently and I gotta say it's pretty good. I mean, as fun as Fallout 4 is. But the shit things about it are the things that make it unique. Namely the multiplayer/online gameplay. It sucks because there is often lag making enemies rubberband, which makes them hard to hit. I think it would be better if you could play it entirely offline by yourself.