It's just part of their industry's neverending campaign to convince everyone that youth = beauty.
Coming out of HS, I was 5'10", 145#, 28" waist...I had an athlete's tone, but was definitely skinny, probably not far off what these fashion designers are pimping.
I'm a half-inch taller now with a 32" waist, and my trainer this morning measured me at 168.5 pounds and 17.5% body fat. I'm no gym rat or meathead, but I'm still in excellent shape...just more muscular than I was at 17. Personally, I think I'm in the best shape of my life...I look better than 99% of guys my age, and I wouldn't trade my present physique for that of my past for all the skinny designer outfits that would fit in my closet.