Because these things are very very small it usually doesn't make sense to say a mole of people because that's impossible. We can't have a mole of people because we only have 7 billion people in the world. So moles are used for very small quantities of something. For example, we can talk about a mole of atoms which is 6.22 times ten to 23 atoms. But we can't really or it doesn't make sense to talk about a mole of books or a mole of x because I don't think we have this many eggs in the world. Maybe we do, but it's just a very large number.