except that military ships were intially used to defend civilian vessels from predators, they later evolved to have attack fleets and defense fleets for the purpose of taking out the enemys civilian vessels or stopping the an enemy fleet to do the same but many naval vessels today (including the US and other larger countries) are still purposed towards the protection of civilian vessels
except also the scale and damage potential of sins is much greater than that of a simple naval battle
in sins, we have dozens if not hundreds of ships in a fleet, whereas nowadays there are dozens only if you are in a MASSIVE battle, and even then...
as such, to effectively protect your trade routes, a single frigate wouldnt cut it, youd need sizeable fleets to run interferrence while the civ ships get away... although, there will always be a new civ ship entering the grav well as it is part of your trade route so you would really be better off just defending your front lines and placing static defences to protect against a few lucky ships that get through