There’s more nuance to it than that. Beans do have a lot of protein, but the specific proteins they’re high in are less digestible than animal protein, and still not as plentiful as in animal protein. And that’s fine provided that you eat multiple sources of plant protein and are able to eat a lot of it. But then the issue becomes an excess of fibre and, in the case of legumes, oligosaccharides.

Disclaimer: I’m saying this as a well-muscled individual. For the average person, deriving all protein from plant-based sources is probably fine.