why showing up late (or not at all) is rude

 ~A black hole is a place in space where gravity pulls so much that even light cannot get out (NASA)

Some people are mystified as to why their buddies get miffed when they change plans last minute. They cannot understand what "the big deal" is, especially if the change is justified from their point of view.

It is really not that complicated.

When I commit to a date with someone, they now expect me, and are leaving a space for me in their timetable. If I do not come when expected, or do not show up at all, I am leaving them with a hole.

When we show up late (or not at all), we have not only taken the liberty to do what we want with our own time, we have taken the liberty to do what we want with someone else's too, leaving them with a debt of time in exchange for the gift of accomodation.