I get that rule. But why does the door close at boarding time and not at the flight time or some predetermined time like 10 minutes before flight time or when boarding completes, whichever is later.
The door closes when it seems like there's no more passengers so they can hurry up and start working on the checklist. They do this so they can potentially get going sooner in case there are delays elsewhere in the many steps that goes from passenger boarding to plane in the air. Theoretically they could be more rigid about having a time for the door closing, but that could potentially mean there are more actual delays as now they no longer have a chance to gain some time in their schedule to make up for potential delays elsewhere.
If the door closed at flight time then the plane wouldn't be leaving at flight time, it would be leaving at flight time + the time it takes to go from boarding to getting in the air. Flight time is the time the plane is scheduled to take off, which means the flight time would diverge from the boarding time, and then you'd once again be asking why the boarding time is different from the flight time and why they don't let you board until the flight time, rinse and repeat.
Sounds like you're describing some impossible system but that is how it works in the rest of the world. There's a time at which the gates close, and if you're at the gate before then then you can go on the flight. If not then they might wait for you for a bit if you've already checked in but nothing is guaranteed.
They can still go early if all of the passengers have boarded.
I got to Dublin airport once at the time that the gates were supposed to close and they said they would wait for us if we were fast... Got to the concourse and it was the furthest gate possible - the sign said to allow a 15 minute walk! Had to run all the way but we made it.
And there's a time in the US as well, that's called the boarding time. Those there at the boarding time get on the plane, those aren't there at the boarding time missed their flight. Banking on getting on 5-10 minutes after boarding time is gambling that they're running behind schedule.
In that case, every single airline in the US doesn't care about their passengers' time because I can't remember the first time that boarding actually started at boarding time. And when it does, I'm stuck waiting an extra 15 minutes so all the priority and higher class passengers can board first.
