The problem is the procurement for many of these munitions.

We haven't been in an active hot shooting war in some time. So things like interceptors we order a couple dozen a year at most just to keep the pipeline active and our allies restocked. The manufacturers are making them on a slow-line process- think hand machining parts, processes that don't scale up easily.

Then we go from needing 50 a year to 500 tomorrow, another 500 next month, another 1000 the month after that, etc. The companies making them are simply not set up for bulk assembly-line production.
They can start, but it takes quite some time to set that up. Especially when the missile itself was not engineered for mass production.

It's a similar deal with more mundane munitions like 155mm shells. Things like the propellant are only made by a tiny handful of companies as it's a dangerous business with low product demand. So with Ukraine and other wars we suddenly want 100,000 shells per month, and we're literally building assembly lines from scratch.