Because they're at war and we're not. Without a command economy, if we want to bump up our shell production for instance, we need a business that is willing to build a factory that makes and sells shells. But what happens to that business after the Russo-Ukrainian War is over, and the shell consumption dries up? Who buys the shells, and what happens to those investors that paid for the factory? Will they pull a profit, or won't they?
This makes it more challenging to simply "build more shells". It's also why S Korea being willing to sell shells would be very helpful. Unlike most of NATO, which prefers to drop explosives from aircraft rather than shoot them out of tubes when an enemy needs blowing up, S Korea has a massive artillery-based army, and routinely goes through large numbers of shells just over the course of regular training for their mandatory military service. They're the only ones in our entire alliance sphere that could potentially satisfy much of Ukrainian shell hunger without some huge, economically impractical build-up. They have laws against war profiteering, however.
This is why getting Ukraine our jets is so important. Jets are our thing, we have those munitions factories already, nobody will simply go out of business after the war is over and Ukraine stops needing missiles and JDAMs.
Anyways, I assume that's more or less what the article said. It's not a complicated concept.