While general fitness is healthy, competing at the peak level (and sometimws far below the peak level, too) in very many (probably most) sports rewards optimizing short-term performance (which aspects vary by sport) over long term health.

I maybe should've specified "at the top level". Your whole body breaks faster: joints, some organs, lungs. There's an amount of exercise that has a positive effect on people who eat more than they absolutely need to, but at some point you are not doing your body any favours.

Note I'm not saying "being fat is okay" or "exercise is bad for you".