There's a scheduled departure time. Anyone who has flown a lot knows that the time the airplane pushes is highly variable - it might be early if everyone has boarded or it might be late (eg, if ATC or other tarmac movements preluded the push, etc). In any case, it's rarely on the exact minute of the published schedule. What CAN be fixed is 15 minutes before "scheduled" departure. That is the time UA has decided they need passengers to be available for boarding. Are you really expecting an aircraft to always depart on the exact published minute irrespective of anything else going on?

Airplane loaded except one missing passenger who is not at gate area and another aircraft waiting on gate that has arrived late loaded with passengers w/ tight connections? Your solution is to wait for the one passenger and perhaps cause 20 misconnects? There's a reason we don't dictate how our personal flights ought to be operated. UA has a lot more things going on besides a late passenger, a GA, and a door. If you're affected, you automatically discount anything else going on - you just see a door, a GA, and plane that you're not on. Next time you're on a plane that's an hour late and you're waiting on a gate w/ a tight connection wondering "when the heck is that damn plane gonna get the heck out of there?", you might appreciate the fact that they closed the flight when they did (instead of making you wait another 10 minutes).

We've all been both the beneficiary and victim of UA's operational decisions. It all works out in the long run.