While first off, gameplay does always need to trump lore, I would contest the assertion that ships should be constantly moving thematically speaking.
Ships move with relatively high apparent mobility(speed & maneuverability) because the game wouldn't be playable otherwise. People seem to forget that even frigates are actually fairly big ships(crews of several hundred at least).
Even frigates by and large fall under the category of "large ships" that in most sci-fi more or less sit(their engines are usually running, but their cruising speed is negligible compared to the scale of the battle) still while fighters do most of actual movement.
Moreover they are not really of the scale where they would realistically have the mobility to make hitting them with weapons difficult. And frankly if you're not avoiding enemy fire by keeping moving, much of the insentive to be constantly moving is gone- at which point you may as well use that engine power for something more useful to comat(such as more power to weapons, shields, or subsystems).
I honestly think that just because they are the "smallest" non-strikecraft ships, people forget that frigates are actually pretty big. Corvettes and Strikecraft are the only real thematically small agile ships in the game- and they happen to also be the only ships that don't sit still while firing.