That's because the Duke is a non-combat role just like the princess. If we say that levels are only a simplification of showing how 'good' someone is (while what actually matters is experience), then what would happen if you gave a lvl4 person a lvl50-and-above weapon? Would they simply be bad at wielding it because of their lack of experience, or would they simply be unable to lift/equip/swing the weapon at all due to game/mechanics-lock?

Remember the whole cooking thing? If you weren't at a certain level of cooking, you just burn the food to a crisp. You don't produce bad tasting food.

The princess and other soldiers may well be able to learn and get better at weapons and skills, but I don't believe they'll be able to level up to access further skills that were previously locked as such.