Humans typically have 20 baby teeth and 32 adult permanent teeth. Considering that some animals have only one set of teeth their whole lives, and some replace their teeth on an ongoing basis, why are humans stuck with 2 sets?
The answer boils down to fit. We don't have a single set of teeth because our jaws just aren't big enough as infants and children. To have your adult teeth in the body and face of a toddler would be quite frightening. Baby teeth allow us to eat hard foods as kids. And they also act to keep space for the adult teeth that are forming underneath the gums.
As for why we don't get more than 2 sets of teeth, it seems evolution has determined that our normal wear and tear isn't sufficient to warrant another set. However, other animals, like sharks and elephants, are much rougher on their teeth, and so require more frequent replacements.