That's like saying that because a 747 can haul some freight that you don't need or want cargo versions of the plane sans passengers. You could do it, but its not optimized for either. It's why every time they try to make an all-service fighter the thing always ends up compromised up the wazoo and expensive as Hell.
Which is one of the reasons I have never believed Starfleet was a straight up "military" organization. A general-purpose starship like Enterprise makes sense as an exploration vessel, one that can fight if it has to but isn't specialized for combat and spends most of its talents on scientific research and field engineering. Contrast with combat specialists like Vengeance and Defiant, the existence of which is both rare and controversial and hard to justify except in the face of the most extreme threats. That's almost the exact opposite priority of military thinking, where the generalist designs tend to be the more controversial and have the hardest time attracting funding.