They don't earn an MD because they don't go to an allopathic medical school, and they don't earn a DO cause they dont attend an osteopathic medical schooL!
So you know that already, I really don't have the answer to why there are podiatry schools - other than the obvious need for Podiatrists 😉
I think it's just sort of a historical anomaly that podiotrists, who seem to do similar tasks to other medical specialists (such as hand doctors), don't go to medical school. I doubt it's going to change now that it's so firmly intrenched.