Because the two pet classes mentioned are two of the absolute highest DPM professions in game.
An Agent, who has significantly less survivability than both professions throughout the levels, does less damage than both of those professions, even in a setup designed to make the best use of the prof's naturally high add damage.
Basically, if a profession has less defense and less damage than another profession in exchange for better (but not actually usable) healing, then there are some PvM concerns which need addressed.
P.S. DD setup Rifle Agents do less damage than DD setup Doctors.