I don't think "rude and inconsiderate" are the problem for most people. My wife, for example, is often late to social things. But she's also one of the most thoughtful people I know. In a lot of cases, her thoughtfulness is what makes her late, because we can't show up without a gift or something like that.

That's just her situation, but I think there's generally something larger at play. Something more like people feeling the need to squeeze the most time out of whatever they're doing right now, even knowing in the back of their mind that they're stealing from the next thing. I think it takes a certain amount of unnatural discipline to think of the current thing and the next thing as time-equal.