To be late except in unusual circumstances, is a way of insulting the person you are planning to meet. It is as though whatever else you were doing is more important than them. It's OK, you think, for them to be cooling their heels while waiting for you.
It's down right rude. It indicates poor planning, or over-committing.
Years ago I worked with a woman who was 20 minutes late every day. Her excuse never varied: "There was a lot of traffic on Saratoga Ave."