Why do some cultures think it is okay to cut in line?

When I was in Europe, and now living in a fairly diverse part of the US, I notice that there are some cultures that appear to think waiting in line is optional. They will boldly march to the front of the line, with what appears to be no shame whatsoever. Does anyone have an explanation for this? I know there are rude people in every culture, but I was wondering if there’s more to it than that? Is line etiquette different in other countries? In the US, I would say it is generally, first come, first serve, with some leniency for those that seem too frail to wait in a long line (physical injuries, elderly, etc).