I think you have it backwards. The reason people are fasting and doing keto is because it seems to be much closer to how our ancient ancestors lived. Fasting is a natural state many animals, including humans, are well equipped to enter. Not only are we equipped to enter fasting, but it seems to have some very good effects on us when we do it
Of course our ancient ancestors didn't have to "worry" about fasting or being keto, just like they didn't have to "worry" about getting enough cardio and exercise. Yet we do, because otherwise our modern conveniences will keep killing us and making us sick. Fasting and Keto are two answers to the question of "How do we better replicate the way ancient Humans lived?"
Of course our ancient ancestors didn't have to "worry" about fasting or being keto, just like they didn't have to "worry" about getting enough cardio and exercise. Yet we do, because otherwise our modern conveniences will keep killing us and making us sick. Fasting and Keto are two answers to the question of "How do we better replicate the way ancient Humans lived?"