Lore-related question: Since Isaac is a duke, I'm assuming he does have some military training, yes?
Oh this is an interesting question! Maybe some spoilers?
In my mind, the world has been struggling with the side effects of the plague and "treatment" for many generations. Some institutions like military academy and standing armies have fallen out of the necessity to perserve family and necessities with a limited life span being (20-30) years old. Noble family like Isaac's who also had advantage of the immortal blessing, would tutor him in history, combat, diplomacy, management, ect but by the time Isaac came into the situation, the "army" would've already been disbanded and replaced with "others" for protection. Isaac is more a Duke on paper at this point, holing up in his Castle and doing the absolute bare minimum to sustain himself, mainly by overworking poor salty Nate, lol.