You are not driving yourself thus you are not in control of when you get there. Who knows how long the taxi will get to your place, etc.

Due to the transit schedule here I would arrive about 50 minutes early to my therapist. She too practically ordered me to stop coming that early. When I informed her I had no choice she suggested I do something else instead of wait in her office. I pointed out it was -25C outside at the time. Was she going to pay for said activities? If she didn't like me in her office, I said, perhaps she should schedule me according to my availability.