Why navies separate missiles from gun barrels

Did navies consider gun-launched guided missiles similar to systems used by tanks?

Yes, but separate launchers allow much larger and more capable missiles than a gun barrel can accommodate.

Navies considered launching guided missiles through guns, as in some tank systems. The comparison turns on the size of the missile that can be carried. A gun-launched missile must fit the gun through which it passes. A separate launcher removes that particular constraint and allows a larger missile, which is the advantage identified here.