It's more that there are maybe a thousand or so people in your highschool, but 7 billion on planet earth. If a thousand people (more like a couple of hundred the same age) exclude you then there are probably unfair network effects or someone poisoning the well against you, but at bigger scales everyone is a stranger and you get to start fresh.

In those cases I think it's more likely mental illness that stops you from making friends, but if no social group has accepted you after tens or hundreds of attempts then it's possible that there's something you might be able to change.