Because not all martial arts are about being "fit." At least not by Western standards. They're about hurting people and breaking their toys. Often being "fit" can facilitate those goals. Sometimes it's irrelevant.
Peace favor your sword,
Kirk
A person who's got strong muscles and doesn't get winded and is light and fast on their feet is going to have a real advantage in a fight - especially in a sport-style art like the poster was talking about. It's certainly possible for an obese person to be skilled at techniques that can harm someone, but they're going to have a disadvantage over someone in better shape.
Also, just... someone that's exercising even a few hours a week and eating a reasonably healthy diet will not stay obese. They might not look like Bruce Lee, but they won't be 400 lbs. And in a sport-style art, they should be getting in at least a few hours a week of decent exercise. So why are they still obese?