The problem is how do you quickly spin up? You can't take an assembly line manufacturing a truck and turn it into manufacturing a tank. Well, you can, but it's basically you take the building it's in, throw away three fourths of the custom jigs and replace it with brand new custom jigs for a tank. I'm sure there's a small overlap of things you can use for both, but it wouldn't be very large.

Tanks are relatively easy to spin on production of. No nation ever started building a battleship in the time of war that then saw service in that war. Battleships commissioned during war had been started during peacetime or they were scrapped during the war because they didn't fit into the time or they were started under high holds, but they couldn't be finished in time. aircraft carriers are likely to be the same place going forward - you have to make them beforehand.

Things like artillery shells, which need to have the ability to quickly spin up production, but that's relatively easy to do. Which is why we're so low on production. The one thing that's lacking is the willpower to actually spin up production because as we quickly discover you could do it but that means all the things that your people are doing instead can't be done.