Matt48,
Generally the flight crew will be either on board or in close proximity to the aircraft at around 40 minutes before departure. From this point on and especially once passengers are boarding the tech and cabin crew by the very nature of being present have assumed responsibility for the safety of the aircraft, passengers and cargo. Boeing and Airbus publish many checklists for problems that may arise prior to closing the doors and starting engines. Various systems that are running could malfunction. Fire or smoke in or around the aircraft. Imagine if the refuelling hose fractured. What then. A major fuel spilll under the aircraft with passengers on board. Who is responsible? Certainly not the engineers since they maybe else where attending to other aircraft. Traffic staff? No training for this scenario. Airport manager. No training and not licenced to operate the systems. So tell me at ETD-30 who is responsible? The responsibility starts preflight. If as you seem to allude that tech and cabin crew are not responsible prior to doors closed then why is there a myriad of procedures for both crew to attend to prior to the first passenger setting foot on board. Why is the first check list called the preflight checklist? Yes that's right we just walk on and start up. Just like a car. Are you a pilot Matt48?