This is a game-balance mechanic that has existed since the earliest days of BattleMaster.
The most important reason it exists is Because Tom Says So. However, there are justifications. For instance, it ensures that a large realm cannot simply send its huge army to its edge, smash it against the walls of an enemy city a few times, then pull back to a border city and completely replenish its soldiers to be back in the field within a couple of days.
In general, it is part of the balancing mechanics that help to ensure that no one realm can grow too colossal.