a medical doctor is an MD
a scientist is a PhD, he has Doctorate of Philosophy.
The real question is why scientists, people who have completely explored, learned and mastered their fields, allow MDs to use the term "doctor" when all they have done is select a medical specialty and narrowly explored it for a brief time in school before dropping out and entering the working world.
Medical doctors are the equivalent of the labor class, sweating away in factories, cutting and sewing, working with their hands for a salary.
Scientists are the researchers and educators, being paid to think and teach and keep on learning, in an academic or occasionly a laboratory setting, achieving grants and Awards, rather than toiling for a paycheck