Protein quality. Whey (isolated milk protein) is "goat" for a reason. It contains all essential amino acids you want + good ratio of leucine.

For something like chicken, the macros are also noteworthy. 100g of chicken is 23g protein, 2.1g fat and 0 carbs.

Lentils have 3x more carbs than protein, peanuts have 25g protein, 32g carbs and 39g fat. So it isn't food that lends itself to a high (ratio) protein diet unless supplemented with something that got better macros (like meat).

Here's a nutrition lecture for macros lectured by a medical doctor with focus being on diet for weight-lifters, if you want some insight into it.