Not all settlements have enough population to retrain big armies. In eb vanilla, for example, population is spawned in just few settlements to keep the game in planed balance (since Pella goes down to fast otherwise) and that is hard. But not all balance is worked out as it was planed - I still see EB team left AS to the mercy of AI rather than working around and inovate script so that Antiocheia would not fall before 220 bc (which means that AS is done very soon after it).

But most of the settlements, 90%+ of them, just dont have the capacite to retrain anything. I would like the players to see that before they start to belive that every campaing the will play on alx.exe will be ''The Megas Alexandros'' campaing compared to the difficulty.