Because reducing requies lowering body fat and body fat reduces at a predetermined rate throughout the body (predetermined by genetics, age, length of time the fat deposits have been there, etc.). In general, you can not spot reduce! You can grow individual muscles by working them although it is almosrt imposible to isolate a muscle, so the surrounding and supporting muscels also get some work, and there will often be some muscle growth throughout the body when any exercise is performed. This is especially true when you work large muscles (like when doing squats)and stimulate the release of growth hormone and testosterone. Yet another reason why you can't spot reduce. The exercise that you try to do to spot reduce stimulate hormonal changes that make improvements all over the body.