system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do many cultures greet or leave with a kiss on the cheek but not in the US?
gpt
There are a lot of complex issues for this. A few of them are:

1) Personal space. Kissing on the cheek is far more invasive than even a hug. Americans have developed a larger personal space bubble than many places due to having more physical space and being less crowded in our cities and towns. Because we are less crowded invading someone's personal space without permission is a much bigger insult than other places. You will also notice that the few cities that are highly crowded like NYC and LA have a decent percentage of the population that do use the kiss on the cheek as a greeting. 

2) Propriety. Anything to do with sexuality is a bit more taboo in the US due to our history with more conservative religious groups. As such a kiss on the cheek, although being very mild is still too sexual of an act for it to be common. And even when it is not considered sexual, it is considered something only granted to relatives (like grandmothers or aunts with young children) and so is considered improper to do as it is claiming a level of familiarity you have not earned with someone.