Chickens evolved in China alongside bamboo. Well bamboo only blooms once every x years, depending on the type. They release seeds which cause the chickens to multiply like crazy. Eventually, chickens evolved so that getting overfed meant making more eggs, which was a good source of food. About 6000 years ago, chickens spread from China to the world, making them a great source of eggs. Most other animals only lay eggs to reproduce, and only every so often. Chickens just do it 24/7 given the right circumstances, which are easy to produce.
Other people have pointed out the facts on cows, but basically, the butter fat separates out easier than other milk sources, meaning butter is easier to make from it. Butter is important, great for cooking. The milk is just a plus from there.
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