How is tempera paint made? Why are there many examples of tempera throughout history?

Tempera paint is made out of egg yolk, water, and powdered pigments (stones, sticks, bone, and earth) but you can also add food coloring. I'm not sure how to answer the second question, but I will give a guess ... I guess there are many examples of tempera (paintings?) because it stays for a long time.

Tempera is made by blending egg yolks with water, pigments, and occasionally other materials, such as glue.