Because unfortunately mags want 'perfection' nowerdays and even the most beautiful person is not completely perfect.
Also models nowadays are very underweight and being underweight means that she will have bad skin, dark circles under the eyes and a visible ribcage and spine, all that needs to be airbrushed and smoothed away to make her appear beautiful.
Even if the photographer is 99% happy with but you can guarantee he'd still tweak that 1%.
They also make the skin 'glow' with a unnatural perfection and get rid of ALL blemishes that are natural in all skin tones. Nobodies skin is naturally as smooth as you see in mags.
Perfection doesn't exist. So I wish they would stop making it the norm and distorting young girls (and boys) aspirations of what beauty is.
Personally I prefer normal pictures.