People need meat in their diets to get a healthy amount of protein to build muscle. Now in our society we recognize that some people, either for preference, idealism or cultural reasons chose to be vegetarian. For these people we have supplements in soy and almond products, or even supplement vitamins. But in Equestria generally everyone is vegetarian and have no need of using meat as a source of protein. So they aren't going to have a lot in the way of supplements. In order to make up for the loss of protein in our diet, we would have to eat a lot cheese unless eggs and fish are an option.