It seems to me that in movies and TV, if someone is leaving, they’re “already late”, or they have to hurry if they’ll make it in time. In the same way, whenever someone arrives, they’re always sorry for being late.
Is there a reason for this that has to do with scriptwriting?
Good excuse for exposition? “Sorry I’m late but Monica, Rachel, and Pheebs’ threesome kept me up till all hours.” “I gotta go–I’m late, and Dad’s already mad at Niles and me for lying about that therapists’ convention to get out of the family reunion.”
I don’t think that’s true at all.
Plus it can provide dramatic tension. If there’s no hurry for the protaganist to get somewhere, why should I, as a viewer, care about the situation?
An extreme example is the bridge scene in Monty Python and the Holy Grail. The Bridgekeeper asks questions, and with a wrong answer, the knight is tossed to his death. But a right answer? “Oh! Okay, off you go then.” The knight is suitably surprised, and it works in that movie, but take that sort of thing in a seriously tense movie, like a Bourne trilogy. It won’t work.
Yeah, I think there are plenty of movies and TV shows where people show up at an event on time. The OP probably doesn’t remember these because there’s no point in a character saying “Here we are, on time for the party!”
For movies/TV shows where people are shown scrambling to get ready in time, this usually seems to be an opportunity for comedy. There’s nothing funny or even interesting about getting ready for an event with no problems. If characters are in a hurry then they can be shown getting worked up and making silly mistakes.
And it provides a reason for the phrase “Fuck, fuckity fuck!” to be uttered.
As far as arriving late, it can be a convenient way for the writer to have everyone talk about the main character before he/she arrives.
Plus, the fact that whatever’s going on can’t really begin until a particular person arrives, even if they’re late, shows their relative importance.
Actually it seems a lot of movies are based on someone getting somewhere early, maybe cathcing his or her spouse with someone else.
One of the most egregious examples of this (and it was, frankly, an egregious example of several other negative trends in moviemaking) was the obnoxiously stupid Navy Seals movie.
The characters train and plan for their missions, and each time right at the start of their mission, they look at their watches and proclaim that they’re “out of time” and have to hurry or change the plan. One time they plan to parachute into water, swim ashore, and then do a whole bunch of stuff…the moment they swim ashore they’re all complaining about how they have no time. Nothing has happened to throw the schedule off, mind you.
Who planned this cloister-fork? It was you, wasn’t it, Lieutenant? And you planned the LAST one and we were out of time at the start of THAT one too! Maybe you should plan on taking a little more time to get to your missions, idiot.
Even a minimally-competent scriptwriter who wanted to use time pressure to increase dramatic tension could have at least explained it – an expository sentence like “That underwater minefield wasn’t on the intel report! Now we’re late!” Or, a bit better, SHOW a reason, like the swimmers having to lie low because of patrol boats. But to just have the characters always be out of time as soon as they get started makes them look like bad planners, not kick-ass soldiers.
I dislike the movie because there they are in the damned church, full of friends and family, celebrant at the ready and they don’t do the 5 minute wedding … thereby doing the fiancee out of any benefits and death benefits … as fucking if … [and we are not going into the odd geography in the movie that does not exist in real life…]
In the British show Coupling, Susan was always apologizing for being late because the women who the character was based on was always apologizing for being late in real life.
You know those miraculous parking spaces that always seem to crop up right in front of the building where the main characters wish to go? Well, when they’re late for something, it means that, offscreen, that parking space DIDN’T appear for once and they actually had to go find a parking garage a few blocks away and hoof it. Sometimes the Parking Gods can be rather fickle.
Or they forget their sacrifice to Asphaltia, the Goddess of Parking Spaces.
“No time to explain; we’re late!” Cuts down on exposition, and explains why character A never told character B the important information that she ought to have mentioned immediately.
Also gives a sense of urgency to the most mundane scenes.
Because they were in make-up.
Okay, more serious answers, by having the characters be late it opens opportunities for exposition.
“Where’s John? I would have expected him to be here by now.”
“There was probably an emergency at the hospital.”
Opportunities for character conflict.
“Where’s John? I told him to be here by six. He’s so irresponsible.”
“Now dear, calm down. I’m sure he’ll be here soon.”
Opportunities to avoid trivial dialogue.
“We’re here.”
“Thank god. No time to stand around. Let’s start the wedding.”
And opportunities for a dramatic entrance.
“John, where have you been? We’ve all been waiting for you.”
“Sorry, I met a couple of stewardesses on the flight and got out of bed late this morning.”
Don’t intend to hijack but I always notice on Law and Order (doink doink), whenever the cops go out of the office and talk to non-suspects, the person always ends the coversation with: “Now if you’ll excuse me I have got to go blah blah blah.” I guess I just have too much regard for authority and if I was being questioned by the police I would wait until I was dismissed! 
As far as people showing up you have 3 ways of starting the dialogue:
Awkward.
Not so awkward. But it often means someone is showing up late.
But even better:
This can be a lot a more natural but requires a better effort on the part of the writer. So the 2nd method is the fallback.
Similarly to end the bit. People just stop talking, someone leaves (for a good reason like being late), or you just end it with something like a pullback shot while the dialogue fades out.
In your standard 1 hour TV drama, there’s a big push to add “action” to every scene. People coming and going does that.
Yeah, Python made fun of these kinds of things years ago and yet they’re still done.
As much as I love Back to the Future, I’m always annoyed at the end when
The car won’t start on time, but he gets it going later and makes it anyway. This is in the precisely coordinated plan to have the car going exactly 88mph at a particular point on the street when the lightning strikes.
He goes back only an extra 10 minutes. There seems to be no reason for this than to provide more tension. Why not go back a few hours early?
Think, McFly. Think.
Maybe the writer has a wife and/or teenage children and can’t imagine any way to arrive other than late.
I was just thinking that in real life, it is rare for me to meet a group without most of them being late.