Because so many people are confused about it.
Psychologist: has a PhD, EdD or PsyD (doctoral level); may specialize in terms of patient age or condition and theoretical approach; uniquely does psychological evaluations including intellectual, achievement, personality, and projective testing; may prescribe psychotropic medications in some states with appropriate licensure and education (not Utah)
Psychiatrist: has an MD or DO with specialized training in recognizing and treating mental illnesses; prescribes psychotropic medications
Social Worker: Masters degree in social work (LCSW)
Marriage and Family Therapist: Masters degree in Marriage and Family Therapy (LMFT), trained in systems theory
Clinical Mental Health Counselor: Masters degree (CCMHC, formerly LPC)
Psychiatric Nurse: RN with specialization in psychiatry (LPRN)
All must be state licensed to practice. Unfortunately, in the past and as far as I know even today, anyone else can hang up a shingle as a therapist, or counselor, or life coach, or energy healer, or whatever, with no oversight or protection for the public.
There are many nuances in terms of theoretical orientation and specific techniques used. Most professions have national and state associations (and web sites) which can provide more information. The state Division of Occupational and Professional Licensing can also be of help.