When you raise an army, you are taking all the fighting age men from your lands into it. If 50%, 60% or 70% of that army gets destroyed in the war those men are gone, husbands and fathers are not coming home.
This isn't Command and Conquer where you can just create soldiers out of thin air. If you devastate the fighting population of your lands, it's going to take years, decades even for the young boys left behind to age up and replace them and your population to recover. Certain councilors can speed the process up a bit, but it's still going take years.
It's why going to war should never be a first option or a decision made lightly. Even a successful war can have disastrous long term effects on your realm.
I will admit, a few setups could use modification to compensate for base game mechanics. City commands like kings landing for instance could use either an event or a decision to bolster the forces. Since they're presumably reliant more on professional guards more than levies, perhaps an decision to spend gold hiring in new blood and trainers to give a large levy boost or reinforcement boost, or upon being granted, have an event to refill levies(With a cooldown to prevent abuse of course, while not ideal if someone dies randomly 2 seconds after the grant, a 5-10 year cooldown on the event would be reasonable to prevent cycling commanders after a major siege for instance).
However, this is more a suggestion forum thing. In general the fact that the game of thrones mod takes the whole levy thing both very seriously and too an extreme is certianly interesting. Since outside of tribals historically a fair amount of fighting age men would of been left at home either because they were too expensive for the lord to justify paying or more likely they just lacked the armory stock to give them basic weapons. Upkeeping a stocked armory with properly maintained weaponry in peace time does take a fair bit of manpower both skilled and unskilled, not just for repairing checking an replacing the weapons, but for ensuring the armory they're stored in is properly maintained and also to ensure that resources like wood and iron and oil sources flow in order to supply these things.
Even a simple sharpened stick of a wooden spear is a surprising amount of effort, since you have to properly form the wood to make a straight stick, then sharpen it to a good point, most likely fire harden it so it's battle ready, then you have to prep and age the wood so it doesn't just rot instantly, then when it's stored it needs kept rather dry since if it gets wet it's actually more likely to become useless than an actual steel sword which you can repair the rust on, a rotted stick is just lost.
That said, if not for A: how many youtubers drop their levies at sea because "Screw those lost guys" and B: The fact the free cities have a hellish time actually not dying horribly after their first war, I'd say the system is quite well made and interesting in how it forces one to think ahead. It's just that neither the AI nor youtubers tend to do that with their levies
'A Game of Thrones' is a user modification for Crusader Kings II not affiliated in anyway with Paradox Interactive or the rights holders to 'Game of Thrones' and 'A Song of Ice and Fire'.
This modification and it's team takes no income or profit, whether it be through donations, advertisements or any other form of revenue.