Let me put it this way.
A typical CK2 empire might have two or three separate armies to move. Generally, however, this number will actually go down during the game (assuming no blobbing into space) as supply limits increase due to tech.
In EU4, a really large empire with the right ideas might be able to support 5 free generals, plus an heir and a ruler. That means an effective maximum of seven armies which are going to be of any use in combat at all. Sure, you can split down to smaller forces, but doing so in order to gain a combat advantage is pointless.
Expecting the player to pause frequently and give movement orders to each of their armies in those games, or to be situationally aware enough to know roughly where all their armies are and when they need new orders is fine. It doesn't break the flow, and generally takes only a few seconds at most.
In a game where you have potentially hundreds of separate "armies" (or divisions in this case), that kind of situational awareness is not possible for most people. Even just giving basic orders like telling an army to get into position or launch an attack may take several minutes of the players time, during which they will be paused and clicking. Getting new troops to the front lines, again, requires the player to pause and spend several minutes clicking. Player errors, like having too many troops cross the same region and suffer attrition or forgetting to give orders to troops who have completed their current movement become almost inevitable, because human minds are not designed to simultaneously track and remember the position and status of hundreds of things at the same time. Computers, on the other hand, find that very easy. What they don't have is the kind of pattern recognition skills or complex judgement required to take advantage of opportunities or to exploit game mechanics, which is where human players excel.
You can talk about micro being the whole point of the game, but the vast majority of the moves the player will be microing are predictable, or have no great impact on the broad progression of the war. Making the player click through those movements every few seconds is pointless busywork which slows down the "flow" of the game while granting no real sense of achievement to the player. Letting the AI automate those movements allows the player to concentrate on areas where there are actual decisions to be made, where their influence can actually have a determinate effect and thus where they can be rewarded for being clever, which is ultimately the whole point and the reason people play games.
