The average person is going to have a hard time getting through two cans of chickpeas, which are some of the highest almost complete protein sources when it comes to beans and legumes. As you can anecdotally see with most vegans it’s almost impossible to get the required protein using that diet. Even Indian cuisine, which is the closest humans have come to perfecting vegetarian dishes, heavily, heavily supplement with dairy products to make up the deficit.
Then don't exclusively get legumes via cans of chickpeas? It's just a weirdly specific and inconvenient strawman you've latched on to. Rice and beans, lentil soups or curries, black bean burgers, seitan, tofu, nuts. There are just so many ways to get enough protein (with "enough" being "way less than most Americans get").