You don’t have to brush your teeth. It’s not a mitzvah mid’oraysa. We dont’ brush our dog’s teeth (course, she has horrible dog breath).
However, you are required to take care of yourself. There are many stories of people who neglected their teeth in their youth, and it came back to haunt them in their old age in the from of dentist bills.
If Hashem had given us built-in toothbrushes, then I don’t know how I’d kasher my mouth for Pesach if I can’t just throw it out and get a new one. (Plus, we’d have to change it every three months – could you imagine the pain of surgery?!)