The staple diet on many hispanic nations is yellow rice, beans, and corn. Many of these people are poor and can't afford to purchase meats on a regular basis.
All meat and/or food products made from animals are considered complete proteins. A complete protein source contains enough of the 9 essential amino acids to allow the body to make the other 13.
Most plant sources lack some essential amino acids, and therein lies the trouble with a vegetarian diet.
Evidently, yellow rice, corn, and beans together contain all the essential amino acids, thereby allowing for complete protein without reliance on meat or a meat byproduct.