A Really Stupid Question?

[M Astronomy]

I'm pretty new to this whole astrophotography thing. I do know though that you stack lights, darks, flats and bias frames to create one image. I realise that you must take several darks, flats and bias frames and then they are put together to create a master dark, flat and bias frame. But thinking about it what I don't get is if you are imaging one target for hours on end, getting practically the exact same image, why don't you just duplicate a 5 minute exposure 12 times? Boom there's an hour of data in 5 minutes. You don't need to wait for ages for the clouds to clear for a few hours. You only need a short amount of time. So why do you need to take multiple shots? There must be a very obvious answer to this question I'm just missing. Help me out here, my brains getting very confused!

[Victor Boesen]

It's because the noise in the image changes from frame to frame, so if you copy and paste the same image, then the noise would be in the same places, therefore there is no data to remove the noise, because there is no image where the noise is different.