R55, I can relate.
I am that punctual guy partnered to a chronically late man. We get along wonderfully, have a great life together, and manage expectations with each other well. I have my own issues, and he tolerates them well. One of his issues is poor time management.
We've been together six years, and he has only been very late to meet me TWICE. On both occasions, he showed up extremely late, with no text about what was going on. I gave him a lecture, and sent him home (or went home myself), telling him I was canceling our date. He got very upset both times. That was five years ago, and he has never done it to me since.
Apparently I've had a positive effect on his punctuality. When meeting mutual friends, he/we are usually 10-15 minutes late, and it drives me nuts, but his friends tell me that I'm a life saver, in that he was always 45 minutes late, like clockwork, before he started dating me. They wonder how it was i was able to whip him into shape. But the way I see it, there are worse problems in the world. He's good to me, no substance abuse issues, no violence, no cheating, no meanness. Just a strange tendency to get very wrapped up in something that he thought would only take a few minutes.
Having been with him for years, and sharing a home together, I see it in action every day. I honestly don't think he's believing anyone else's time is less valuable than his - he is actually more polite than I am, believe it or not. He really gets sidetracked, though, and when you call him out on it, he apologizes and feels awful. These days, he is more aware of this tendency, and tries ahead of time to do better, and he does. Not perfect, and probably never will be...but there you have it.
What sidetracks him? Well, we will be getting ready to go out, with a goal to get to a certain place by a certain time, and he will suddenly remember something, like moving the clothes from the washer to the dryer, or turning on the dishwasher, or leaving windows open too long on a cold day...he'll go to correct something 'quickly', and he'll notice something strange in the process, and become very focused on it. He'll carefully examine how that shirt he put in the washer didn't successfully have a stain removed from all the treatment, and he will get upset about it, and put more stain remover on it - then he'll look online to find a better technique, because somehow he thinks that the clothes being WET will be the best timing for this, so it's now or never. Then he's on an internet spiral, getting confused about not finding the answer he thinks should naturally be there. He knows he's getting delayed, but he thinks it's really important because otherwise something will be 'ruined', and he'd rather risk lateness than ruin or waste something he worked hard for. That's essentially the issue. He doesn't know to plan for those random situations, or to just let them go, or to simply 'do less' in the first place (that 3rd load of laundry could have waited until next week, for instance). Meanwhile, I'm downstairs with my shoes on, waiting, and losing my mind...and sometimes going back upstairs to investigate and convince him to let it go, or provide a solution that gets us out of the house sooner. Many times, he'll go along with me, not always...
So no, I'm not accepting the theory that he's an asshole. The man has taken better care of me than my own family has, and he is loved by all our friends. He's a good person, with a bad habit that is slowly getting better over time.
Things are often more nuanced than we think. Sure, my story may not be the same as others', and some people doing this may in fact be arrogant narcissists, or divas trying to make an entrance. But sometimes there are other things at play.