Then why do so many people have living wills?

From a legal point of view, a living will is not really very like a will. One's will contains the directions for distribution of one's property after death. In short, the key focus of a will is financial.

By contrast, a living will is one's list of instructions regarding medical treatment when one is unavailable to consult (i.e. unconscious). Do-not-resuscitate requests, and the circumstances when one does and doesn't want particular intense medical interventions. Also, who should make decisions when your pre-made list does not address a particular circumstance. When one is creating a living will, financial considerations might play a part, but the key focus of a living will is medical, not financial.