Because that demand is sporadic, and never constant. We've been seeing these on and off shortages for the last 13+ years. When the demand drops off, (and it ALWAYS does), theses ammunition factories would end up having millions of dollars of equipment sitting. Not to mention, they would be forced to lay off the people running them. That's just not good money, or employee management.
When this kind of thing happens, they are better off to run the equipment they have around the clock, and put everyone on full overtime to do it. And produce as much as they can, as fast as they can. Which is exactly what they're doing now. Sooner or later this will end. Usually when there is a major shift in the political winds. And that always happens as sure as the Sun rises in the East. It's only a matter of time.
What I don't understand is how many of these ammo shortages do we have to go through, before people see the light, wake up, and start stocking up when it's cheap and plentiful?..... Like it all was from 2016 to almost 2020. I keep hearing how people, "couldn't afford to" when 9 MM was $9.00 @ box..... But now, all of a sudden, they can afford it at $35.00 @ box. Not to mention burning 3 tank fulls of gas to find it. It all makes no sense.