Because no single way of eating works for everybody.
One of the most attractive things about belonging to a religion is the sense of community it gives. You meet regularly with people who, theoretically at least, look at life in similar ways. Sometimes you have certain practices you do, or don’t do, and regardless of what you think of those practices, they serve to solidify the community and make all its members feel like part of the whole. And of course, it’s all based on what the members of the religion believe.
Sometimes, people decide on food diets or regimens for similar reasons.
Paleo? No grains or sugar or legumes or alcohol or dairy – paleo dieters don’t believe humans are evolved to cope with those foods. Vegetarian? No meat – vegetarians (mostly) believe eating meat is unnatural for humans. Low fat eater? Low fat dieters declare their belief that keeping fat out of their diets will make them thinner and improve their heart health. Raw foodist? No meat, and no cooked food – raw foodists believe cooked food encourages the buildup of toxins in the human body, and is responsible for a host of health problems. Fruitarian? Fruit only – preferably fruit that has naturally fallen from trees and bushes, rather than picked – because fruitarians believe that eating the fruit of any plant is the healthiest and most ethical way for human beings to eat and be friends to the planet.
You can see how many of these beliefs are contradictory to each other. Proponents of each diet will show you research that explains their beliefs. The bodies of research contradict each other, too. But each of these diets give their adherents a sense of community. These food-commnities are all focused on similar goals, and on achieving them the same way. It’s human nature to want to break up into groups. It’s a somewhat destructive part of human nature to think that one’s own group is better than all the other groups.
But when you come right down to it, food choice – like religion – can’t be universal. Does a certain belief system work for you? Then follow it. Does a particular diet work for you? Then follow it. Are you healthiest when you take elements of different food-choice “doctrines” and adopt them? Then do that.
But we must all keep in mind that what we, as individuals or group members, believe about food and health may not be necessarily true for everyone. Science has a long way to go… and humans are dynamic organisms. We’re not all the same.