Maybe because the US is such a big country, and people are used to having plenty of space for themselves, they tend to hope for the same while they are on vacation. Although they have no issue being friendly and talking to strangers, they usually value that personal space and want to maintain it as much as possible. There is a good balance between being friendly and keeping a distance.
This may be a bit of an extreme example of people keeping their personal space, but you get the idea!