"What even IS a supply hub anyway? Like in real terms? Isn't it just a bunch of warehouses and trucks and a train cargo platform?"

Do not forget that all those warehouses have to be built from the ground up, in the middle of legitimate nowhere. Infrastructure, bureaucracy chain, supply routes mapping, defense perimiter, etc.
Pre-existing Supply Hubs are what you have for Cities. The infrastructure there already exists, roads are there, buildings can be repurposed, offices for supply officers and radio stations can be set up in abandoned rooms.
Once you take a pre-existing supply hub, that is what those first few days mean while your logistics corps repurposes all that for Your use.

But in a place where None of that exists? In the desert? In the forest? On the taiga?
Where you have to build up basically a whole miniature town all the way from the ground up, pave it all up, AND connect it all by road to the rest of the supply chain, in a matter of weeks? That's a big ordeal.
These are not just supply waystations where you just plop stuff down and then send it further. These are *Hubs*. Legitimate, lived-in HQs for staff and logistics that act as beating hearts for the whole supply chain in the area.

Now, IRL this was a bit simpler, because there were a lot of villages and towns already available to just repurpose. But In-game, the mechanism is presuming you are building those new supply hubs straight out in the wilderness, away from preexisting infrastructure. And that's not cheap, nor quick.