@Gamecock's spot on. When the plane's at the stand, it's only got its APU running; enough to run the electrics, and push air through the cabin. Only when the main engines have started can you check hydraulics, fuel pressures, the engines themselves of course, plus all the control systems and the aircon packs.

By the time the pax are starting loading, all the weight/balance calculations have been done, the fuel amounts have been worked out (although fuelling may still be in progress), the routes have been entered into the navcom, and all the checklists that can be worked, have been. There's not much the flight crew can do until push-back.