It's not very rare at all. It's rare if you compare it to grains, and while quinoa is not a grain, it is usually used instead of a grain. Many beans (e.g. white and black) and vegetables have a complete amino acid profile.
Even potatoes have a complete animo acid profile. Not in the most optimal ratio, but 10 potatoes/day gets you enough of all the essential amino acids. In fact you can live exclusively on potatoes if you add in a source of Vitamin A (like carrots).