- Do you imply that industrial farming is the only way to farm? If not, why wouldn't we develop alternatives instead of advocating for abstinence. It's as if you were asking people to be naked in the street because some clothes were made in sweatshops in Asia.
- It's elementary. For a protein to be usable by the body, it needs to be complete : https://en.wikipedia.org/wiki/Complete_protein and in the right amounts.
Vegetables don't contain complete proteins generally, so you have to eat a diversified, complex array of food to equate animal products. Animal products are complete, and their profile is very close to what the body needs.
Another problem is that “high protein” vegetables usually come with plenty of carbs, or fat in the case of nuts. If you have a high-protein diet because you're a young, active male, you'll end up increasing your caloric intake too much. For instance, in the case of Quinoa, you get 39g of carbs for 8g of lower-quality proteins.