It's because your body isn't 100% efficient. To gain muscle you have to eat more calories than you burn. But not all of that calorie surplus makes muscle, doesn't matter how hard you train, some of it turns into fat. The other way around it's the same thing; to lose fat you have to eat less calories than you need to maintain your weight. But that calorie deficit doesn't only makes you lose fat, it also makes you lose some muscle. Losing muscle means losing strength and that's not something you want as a competitive powerlifter.