A doctor is a medical practitioner so why do you call Ph.D holder and yet he not practicioner?

While a medical doctor holds the title "doctor" due to their expertise in the field of medicine, individuals with a Ph.D. also use the honorific title "doctor" in recognition of their academic achievements and scholarly expertise within their specialized field of study.

Assistant

The distinction between medical practitioners and other individuals who hold Ph.D arises solely due to a difference in expertise and professional domain and does not diminish the significant accomplishments of both types of scholars with doctorates.