The supply system for Europa Universalis (which is part of what I referred to as "other Paradox titles") is actually very barebone, consisting almost entirely of "you can only keep a certain number of troops in a province without attrition", where the number of troops depends on terrain, friendly/enemy territory, etc. Hearts of Iron has actual supply chain management.

As for Civilization, I've always treated it as a battle chess game rather than an actual strategy simulation game.

That's actually pretty accurate for the era EU simulates though. Nobody could supply an army of any size by any method other than foraging, aka buying all the food and forage in the area around the army.

As you know, the maximum throughput of any connection is the buffer size divided by the round-trip time. If you're using horse-drawn carts to move food and forage, the buffer size will be quite small; carts don't hold very much and there aren't a lot of them. Horse-drawn carts are slow, so the round-trip time will also be quite long. A number of generals had the idea of supplying the army this way, but nobody could do it further than a few dozen miles because the round-trip times get so high. Before WW1 everyone figured that trains had solved the problem; you would just load your supplies up on big fast trains and send them right to the front. Unfortunately, armies really love blowing up the enemy's trains and railways, so the railheads were never as close to the front as anyone liked and they were always having to be rebuilt. There weren't enough motorized vehicles in the world to deliver all that tonnage more than 100 miles, so the fronts in WW1 were very stagnant. In WW2 there were tens of millions of trucks of all sizes, and the Germans estimated they could deliver sufficient supplies to the army as far as 400 miles from the railheads. Unfortunately, it's over a thousand miles from eastern Poland to Moscow, so that didn't go so well. Also, it's over a thousand miles from Tripoli to the Suez canal, so they didn't have much luck in that theater either.

If you want to read more on the subject, I recommend "Supplying War: Logistics From Wallenstein To Patton", by Martin Van Creveld. Great book.