I'm not certain, but I think flight crews start to get paid when the door is shut. Even if they "find" a cause of delay after the door is closed, they get paid. But if the pax wait in the terminal, they do not get paid because they are not yet "on duty".

That is why they rush you to get on the plane and take your seats, close the door and (then) discover a problem that takes maintanence 1 hour to fix. They get paid for that delay. But if it was "discovered" before the door was closed, they would not get paid for the delay.

True, it would be more comfortable for the pax to wait for an hour in the terminal or bar, but the crew would not be earning $$$ during that time!