Because rice and beans together make a complete protein. Link: https://www.livestrong.com/article/351077-the-protein-in-rice-beans/

To reduce the arsenic in your rice, first give it a good rinse. Place the grains in a fine mesh strainer and pour water over them until it runs clear. Cook the rice in excess water, at a ratio of one cup of rice to six cups of water, and drain any extra leftover once the grains are tender.

https://www.allrecipes.com/article/arsenic-in-rice/