This has been mentioned a couple of times in this thread. It surprises me that tardiness for a D&D game can be so important.
I know you've got a ton of replies to this already, but I wanted to come at this from another point of view, and I'm sure you may have thought of it already, but I wanted to explicitly express it.
For some people, D&D is a fun game to play with their friends who they do still see regularly. Hell, I live with three members of my group. I've known another since he was a freshman in high school (I'm kind of like his big brother, was the best man at his wedding, etc.). I've known the fifth member since before he was a freshman, but we're only just now catching up over the course of the past 3 or so months. The newest member has been playing with us for about a month, and I just met him.
Outside of the two newest members (who are good guys, but I don't know them very well yet), I see all of the other members regularly. So, when my roommates decide to leave at ten minutes before the game starts to get food, I cringe, because I don't know if they're grabbing something from Subway two minutes away, or if they're going to spend 45 minutes waiting for Mongolian food.
This annoys me, because as the GM, I'm supposed to set the pacing for the night. That means I don't want people eating during the first 30 minutes of game (I've found that there's way too much, "can I trade you some chicken fried rice for a slice of pizza" going on for the players to immerse, though if I wait about 30 minutes, it's not an issue). I don't want three players waiting for three others players because they didn't schedule their time very well. They could have grabbed that food an hour ago, but didn't leave until the last minute.
The reason this annoys me is because it's inconsiderate to me, as the GM (since it's pretty disruptive to me pacing the session), and to the other players who showed up to play the game (since I see one of them regularly, and he sees the other two regularly). We're good on socializing. We have a "Game" group of Facebook that we post in, talking about next session, posting maps, sharing backstory. We used to talk about mechanics when I was still testing my game. We socialize on other days besides game day.
Thus, while it might be within your social contract (essentially your group's expectations) to be okay with people being late, it's not for everyone. Not everyone is lacking in social interactions with their group. To that end, I'm not sure why it's surprising to you that tardiness can be a problem. Maybe this shed new light on it, maybe not. Just thought I'd share my thoughts with you.
It works for your group, and that's awesome. This is why it doesn't work for mine without annoying me. As always, play what you like
