Because the U.S. has a 60% obesity rate and you look awesome compared to the average person.
But being skinny sucks. However it can be a compliment. Getting skinnier is a goal for a lot of people. You never hear about someone trying to get fat. There is the reality of too skinny also being unattractive and some a lot of people wish they could gain some weight.
If the obesity rate was actually that high this country would be in even more trouble than it already is. You're thinking of the percentage of "overweight" people. The obesity rate for the US is about 35%, still making the US the world leader, but "overweight" and "obese" aren't the same thing.