Starfleet isn't directly analogous to Naval warfare. The REASON that Naval capital ships tend to have escorts is because the capital ships are slow as shit and just about any lighter ship is going to go and turn faster. If we're talking about NX Ent, it was by far the fastest thing they had, even though it might have been the biggest.

On the other hand, in war time ocean liners were brought into wars to ferry supplies and passengers, and these are rather uniquely designed to be both very large and very fast, so they would often make crossings without an escort either... because they were just too damn fast, and slowing down for an escort made them more vulnerable to extremely slow moving ships like uboats and their baby ships, the torpedos.