They are still not complete proteins like every animal protein is, so you need to figure out combinations that can theoretically make a complete protein. And the protein content is still going to be much lower than meat. Protein is already the hardest to get enough of, it's always easy to eat carbs and fat, which people already do too much. And if something is hard to digest, common sense says that it's probably not utilised as well by the body either.

In practice it really makes a big different, to eat 100 grams of chicken for dinner, compared to 200 grams of lentils. Most people that eat plant based compensate with eating more fat to get the satiety per gram of food higher, which leads to an unhealthy diet.

It would be much simpler if plants had complete protein in the same amount as animal protein sources, but it's way off.