Might be a dumb question, but the general nutritional advice I’ve gotten is that eating protein is essential for muscle development and maintenance. Evolutionarily speaking, why? Carbs seem to be much easier to come by, and strength seems to be very useful for survival and reproduction?
Nitrogen. Although the air we are constantly breathing contains a very large amount of nitrogen, humans are only able to obtain usable nitrogen from amino acids. Human bodies are incapable of producing new amino acids - we are only able to modify amino acids that already exist. We must consume nitrogen in our diet - whether animal or plant sourced. Nitrogen is pretty much the defining characteristic of proteins, in terms of macronutrients.
Neither carbs nor fats provide nitrogen in a usable form, unless there are amino acids involved.
Carbohydrates, fats and proteins to some degree are all required for building muscle tissue (emphasis on the some, as you could do without one but not nearly to the same degree). Proteins are made up of Amino acids which are “building blocks”. Think of it like building a brick house. You can have the workers (carbohydrates), management (fats) but you still need the bricks (proteins). Like building a brick house, management (fats) can also do the grunt work but tend not to be as effective (on average) as compared to the standard workers (carbohydrates). The workers can build without management (albeit maybe not nearly as well) but they cannot at all without the bricks.
I hope my odd comparison makes sense.
I could explain how carbohydrates and fats assist with muscle growth but that’s a lesson for another day.
Edit: added words
We kind of do, kind of. Carbs help to support glycogen formation, but also the combination of carbs and protein post resistance exercise has proven to be more effective at triggering MPS that only protein alone. But muscles are only composed of proteins as is.
For starters, muscle is built of amino acids, not polysaccharides or glucose. Polysaccharides (or any of its lesser forms) are not going to build contractile proteins, like actin or myosin.
At the very basic level, it doesn’t make sense to question why protein is required for muscle growth and carbs are not.
Carbs were very scarce when we evolved, but evolution is kinda irrelevant to the question. Muscle/bone/lean mass is built from protein, and we largely(as a species throughout time) built it by eating other animals that are made of protein. Survival of the fittest, there’s always a bigger fish, etc.