The idea of having the same game mechanics only with the frontline being "invisible" also would simply be frustrating. Now, imagine you've pushed back the enemy super far into their deployment zone, but a large armoured spearhead got through and is now roving around your rear lines. Realistically, such a force would be easy to get a rough ballpark of where they are, even if you don't have eyes on them. Noise, dust, sigint, etc. With frontlines, you can tell the rough area where they are. Without frontlines, you'd have to pretend that they could be literally anywhere in your backyard, and spread out looking for them. In fact, the only thing telling you that they are even there is the fact that you keep losing points. Now imagine that this force broke up and scattered behind your lines, capturing random parts, occasionally connecting with other bits to randomly cut off your own troops in tiny "pockets". You'd see sections of your forces suddenly become "surrounded" with the morale hit without warning. And you would have to spread recon forces across your entire back lines to keep track of these weird tactics.
Those strange ahistorical game-y exploits are easily dealt with a visible frontline.
Those strange ahistorical game-y exploits are easily dealt with a visible frontline.