A myriad of reasons. Undercoding and not charging a retainer are two of them.
As an aside, here’s an interesting tidbit the doctor shares about not wanting bad publicity for her fledgling practice:
One patient, whom I suspected of faking, even asked me to help her complete her disability paperwork. When I hesitated, her husband threatened to tell all her friends what a bad doctor I was. Fearful of that kind of unfounded publicity, I reluctantly and regretfully complied.