Um. I lived in Atlanta - world's busiest airport - for a decade. You would NEVER see that many flights from Delta to the same overseas destination leaving at virtually the same time. So, I too find it ridiculous, despite "countless" examples which I've never seen.
You actually lived in the airport? Wow.
While I can't say I've actually lived
in one, there are many weeks that I literally spend more time in them than I do my own home -- I have earned over 2 million miles on American (all Domestic -- perm Platinum), 1 million on United (perm Premier Executive), and have little trouble earning my A-List w/ Companion on Southwest the past few years that they've been my favorite (used to hate them many moons ago). I would say that I spend more time looking at A/D screens in US airports in a given week (for those locations I fly to that actually have
screens) just watching the flights come and go than most people spend watching TV.
Atlanta doesn't have massive international demand (in comparison to its' domestic percentage), so it's no surprise you wouldn't see overseas destinations with departures that close. But I can 100% guarantee you that you'd see somewhat similar schedules for more domestic destinations out of ATL. Atlanta has numerous destinations that it originates over 100 flights per day for -- you aren't flying that many flights without bumping them really close together, at some points of the day within 5 to 10 minutes of one another.
It's also common for carriers to fly planes to seemingly unusual destinations very, very late at night, often with flights very close to one another, for the purposes of repositioning aircraft and/or clearing out the major hubs. It's common, for example, in many Midwestern smaller airports for most of the outbound flights to be very early in the morning and most of the inbound flights to be very late at night -- simply because the airlines use them to park their planes at overnight. As a result you end up seeing several flights to ORD, DFW, ATL, all the major hubs all pressed very close to one another between 6am and 7am; and likewise many departures from those hubs late at night, sometimes very close to one another. Perhaps not every 15 minutes, but FAR closer together than the demand on that route would necessitate at 10pm at night. Omaha, for example, departs twenty-five
flights in the morning before it ever sees a single
arrival. Every single one of those planes was left sitting on the tarmac overnight, and most all of them are going to just a handful of major airports, including no less than 3 flights to Denver within 10 minutes of one another before the sun even comes up.