I don't know the details but look at what Slippery_John said in his comment:
> Well yes, but actually no. Not all proteins are equal, they have different quantities of different kinds of amino acids. Humans need 9 distinct amino acids for a healthy diet, and animal proteins are by far the easiest source of "complete proteins" from a single source. It's absolutely possible to get plant-based complete proteins (e.g. soy or quinoa), but often that involves deliberately mixing different protein sources to get a complete balance.
> Well yes, but actually no. Not all proteins are equal, they have different quantities of different kinds of amino acids. Humans need 9 distinct amino acids for a healthy diet, and animal proteins are by far the easiest source of "complete proteins" from a single source. It's absolutely possible to get plant-based complete proteins (e.g. soy or quinoa), but often that involves deliberately mixing different protein sources to get a complete balance.