I did catalog modeling from like 5 to 17. My mom and dad both modeled. My dad did stuff like lawn mower shots and stuff with cars and motorcycles. He is the most successful in the family. Male models are really rare, especially over 30. As you get older the tougher the competition gets. Super intense as a developing teen. If you expect to get paid anything on a consistent basis, slim is in, as is tall for your age. At 14 my career peaked. :) At 19 I rarely get any calls except from biker magazines and semi nude but no sex internet sites.
Because it's the most aesthetically pleasing. That's their job and that's the business. Fat people can whine and whine all they want but no one wants to see cellulite and rolls. You wouldn't hire a blind person to be a pilot or a taxi driver, right? Same deal.