Airlines also pull their planes back from the gate for on-time departure rankings. Even if your held on the ground for a total of eight hours, if you push back from the gate within 15 minutes of scheduled departure, you're on-time according to the DOT.

Also, I doubt that airlines, as a rule, would always want to give their customers the opportunity to get off the flight. While I suppose that each captain may have some discretion, keep in mind that each pax who gets off is likely lost (or potentially lost) revenue.

Indeed, the government has little to do with it. When it doubt, remember this: the FAA/ATC has cancelled a whopping ZERO flights in its entire history. Government gives directives, but it's the airlines that make you wait on the plane or cancel your flight.