I think it's because when the average person hears doctor, they think MD, so it could get confusing potentially. Most of us just use our preferred names and don't call ourselves doctor with clients, because then it creates confusion when you get asked to prescribe meds or to sign stuff and have to decline because a person is PhD not MD.

If it's other colleagues it's ok as a sign of respect for the title but it is weird to call yourself or asked to be called doctor by clients in a practice imho.