Because they're looking for an excuse to let go of the last bit of self control they have. Self-diagnosing with an ED is the perfect delusion for fats. They try all sorts of faddish extreme diets that are not maintainable lifestyle changes and lose a bunch of weight, but because they didn't learn any moderation or self control, they just balloon back up when they go back to shoveling their faces full of crap. Then the fats wonder why they can't keep the weight off, and come to the conclusion that, "Oh, I'm not a disgraceful fat fuck, I just have an ED!" proceeds to eat even more than it did before. It can't be due to their poor choices and lack of effort to make permanent lifestyle changes. No, that would be taking responsibility, and lord knows that fats are not responsible for anything. it's obviously because they have an "eating disorder".