Hah? Weight loss can certainly be attained through exercise. Basically, you need to burn more calories than you take in. You can do that by reducing calories, or by increasing the burn rate. If you keep your calorie intake constant and increase your exercise, you will lose weight, all else being equal.
While this is technically true, in practice it is very, very hard to significantly increase your exercise while keeping your caloric intake constant.
This is simply because you get much hungrier when you're exercising. If you increase your exercise volume while keeping your eating constant you'll feel miserable and hungry all the time. Just like dieting, except you'll feel worse for a given calorie deficit.
You can lose weight through diet, exercise, or a combination. For most people a combination works best but you have more leverage on the diet side than on the exercise side.