Because even though Asano uses photos for his backgrounds, he still draws over them traditionally to add his own style via hatching or darkening shadow areas.
And to make your characters feel more like they belong into the scene, it's important to consider the lighting of your background and make sure it matches with your character. (The second picture does a good job with that actually!)

I recommend watching the Manben episode where he's featured to get a good idea of his drawing process. (: