Because it feels more comfortable to them.

How close people tend to stand to each other in different social situations is a product of their culture. An anthropologist named Edward T. Hall described this phenomenon in 1966 and he coined the term proxemics to describe these social distances.

There are four different levels of social distance:

1. Public space – the amount of space we like to leave between ourselves and strangers in public places when there is lots of room for people to spread out
2. Social space – the amount of space we like to have when doing business or when we are in public places like beaches or bus stops
3. Personal space – the space we are comfortable keeping between ourselves and friends and family.
4. Intimate space – the space we are comfortable keeping with people we are intimately close with.

In North America, for most people, the distances at each of these levels is greater than in China. This means that North Americans will tend to stand farther from other people than Chinese people do. This is probably the result of the differences between the population densities of the two places historically and even today.

Sources:

1. Sheppard, Mark, “Proxemics.” (http://www.cs.unm.edu/~sheppard/proxemics.htm), (28-Sep-2009)
2. Wikipedia. “Proxemics.”  (http://en.wikipedia.org/wiki/Proxemics), (28-Sep-2009)