It doesn't have the calories, someone of my size needs between 80-120g of protein to build muscle. I can get that with 2 pints of full fat milk and a 500g pot of Greek yoghurt, that along isn't enough to build size, your body needs lots of fats and carbs too, just to function day to day. That's even true of me, who spends the vast majority of the time lying on my couch😂.