I totally agree that each person's concept of punctuality is largely based on your culture or nationality. Many cultures have a slower pace of life where exact punctuality is not required or expected.
However, in more advanced nations, business and economy demand that people be on time. Things happen fast in business and daily life here in the US. This is why most businesspeople expect punctuality. I think that this necessity to be exactly on time has trickled down into the average American's belief system. Many Americans find themselves constantly wishing for more time in the day because of the frantic pace and many responsibilities of US life. This leads to much frustration on their part if someone doesn't meet them exactly on time, since they feel their precious time is being wasted. That being said, exact punctuality is usually associated with the more successful cultures.....if only because that's what successful business demands.
My guess is that your parents were very punctual and brought you up that way. Because of this, try not to hate your girlfriend for not being on time all the time. (I'm not saying that you can't talk to her about it and let her know how you feel.) This is how she was brought up, and you shouldn't expect that to change too much. It is part of who she is and will most likely never go away unless she herself wants to and makes a great effort to change. As stated by Jzero, you need to search within yourself and see if this is something you can live with forever. You may love her, but if this drives you to the point of insanity each time it happens.....you have some serious thinking to do. If you decide that you can live with it, stop getting mad or resenting her about it.....it was your choice. If she feels that you get angry at her every time she's late, resentment can build up on her part.