But they don't contain a lot of the fats/acids that come from meat that are hard for the body to synthesize/metabolize. I'm not saying only eat meat/fish, but there's more than just protein.

I did not claim that legumes are some kind of wonder food with the ideal balance of fats, proteins, and carbohydrates. Of course you need to eat different kinds of vegetables/fruit/plants. As for fats, nuts are a good choice. But you can also buy all kinds of oils, if you are afraid that an all vegetable diet will leave you with too little of them. https://en.wikipedia.org/wiki/List_of_vegetable_oils