That is historical. Armies in this era drew their food, fodder, clothing, money, and shot overwhelmingly from the front. Even the ranks were often refilled from local sources as most armies were not particularly loyal, highly polyglot, and often had a few out-of-favor nobles attached. What tended to come down the supply lines were new/more guns, powder, and most important of all: orders and communications. The combat effectiveness of most unopposed armies tended to rise for a period of time as plunder and looting often brought in at least camp followers who helped bulk out the army.
What is blindingly ahistorical are the instant communications. Generals lived in blind terror of walking into an ambush if they got ahead of their communications. And in the real world, alliances and truces were mere suggestions. Armies could, and did, blunder into situations where their allies had flipped or a truce was violated.
And for both communications and supplies, the overwhelming concern was not overland marching, but riverine transport. Pretty much every celebrated siege of the era was either of a riverine fortress or a forward port city with a good harbor.
Assaults should definitely be more viable, but nonetheless ZOC is just something that does not match a period of history where Louis XIV, explicitly built up gunpowder depots on the far side of enemy fortifications and marched to and right past his primary siege target (or another fun time where he bought his powder from the town he was actively sieging).
"Fortresses are equally useful in offensive and defensive warfare. It is true they will not, in themselves, arrest an army, but they are an excellent means of retarding, embarrassing, weakening, and annoying a victorious enemy."
-Maxim XL of Napoleon Bonaparte
This brain bug where the only means of making forts useful is ahistorical ZOC should really die so we can go for something superior that Napoleon would actually recognize.
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