Space is huge, but workers gotta be fed, ships gotta be manned, ship construction materials gotta flow to where they're needed, and so forth, and so forth. Half of spycraft is using what you can see to figure out what they're not showing you.
As regards the turret issue, you also have the problem of what you're going to do with the cheats in peacetime: while the rest of your fleet is out showing the flag and doing whatever work peacetime warships do, your cheats have to stay out of sight. If your peacetime work is important enough, that means the more cheats you have, the fewer ships you have for the peacetime work. If the enemy can make a fair guess about how many ships you are supposed to have and can use spycraft to guesstimate how many ships are actually operating visibly, that is suspicious in itself. Unless you're completely insular and can assure yourself that the enemy cannot penetrate your society to gather information, you are left trying to limit the extent of your cheating so it can go unnoticed - and I don't think turrets are the kind of thing that would make a major strategic difference if you were limited to the levels of cheating that might escape notice or that could be comfortably ignored for the sake of peace.
