You never need to make 100,000 shell a day, you don't just fire them from the factory, which mean you need someone or something to transport 100,000 shell a day from the factory floor to the frontline. Because they don't magically appear on the frontline if you make them, it's not Field of Dream "if you make it, they will come"
The volume of fuel you need to just bring 100000 shells to the frontline everyday would be huge. Otherwise, it would just be stuck somewhere near the factory floor.
The need for supplies was not dictated by how many you can make, but how many shell you need and how many shells you can supply to the frontline.