Or, "Because it's (the 8x10) the equivalent of a 1,300 megapixel camera." * (Or whatever the equivalent is for the camera you're using.)
Or, "Why are you using a phone to take a picture? Do you hammer nails with a screwdriver?"
* A scaled-up rough estimate from a generally assumed 35mm equivalent of ~20-24 MP. Like Bluto Blutarsky in Animal House, in the given context accuracy don't really matter. He's on a roll...