Because emerging adults are in their sexual prime. We are instinctively driven to reproduce with the best candidate. There is no logic, no conscious selection. It's not intellectual viability that gets the job done its ones physical attributes that is the drive at that stage in life.
That's a false statement.
The most important part, it's a personal decision so it would be defined as individuals. Then, usually these kind of things exist into teenage world not in adulthood.
But also, some people confuse the matters, maybe you're one of them as well.
Looking to have a healthy body and metabolism and try to be as fit as you'd enjoy has nothing to do with looking for physical attractiveness.
I do gym and i don't care what others thinks about my body in particular girls, i do gym and try to keep my body fit and i do that for myself and my life not for girls or just being attractive. I just wanna be as sharp as possible as a human being in my own skin as a man.
In a mature world, the "attractive" sickness doesn't exist. Look around, you'll see that.
Because for the most part ... They are still immature and shallow.
Because at that age you haven't learned what is truly important yet. You are focused on instant gratification, and not really paying attention to the long haul.