because its not JUST about protein. the body needs X amount of calories to maintain its current weight.
as soon as you start tearing down muscle, you need to repair it, and that takes calories.
If you add 500 calories when you don't need it, you gain fat.
If you don't add any calories and just up your protein, you can add muscle, but its slow...painfully slow. instead fo a couple pounds of muscle a month, you're looking at several months to gain a pound of muscle.
if getting results in a reasonable, non-frustrating amount of time is important, you should go with bulking and cutting cycles to optimize results.