I think the shortage is mostly due to a lack of a desire to make big changes to resolve the issue.
If a meeting was called of 10 big tech/manufacturing companies and they were each told they'd be hansomely rewarded if they managed to make large volumes of a missile design, and that all IP/Patent laws would be suspended for the purpose, I guarantee you that there would be massive stockpiles within the month.
We have no idea what a major economy like the current US would look like if it went on a real war footing. According to this [34] we spent 40% of GDP on WWII (equivalent of $5 trillion today) - that would be almost $10 trillion a year now if we did the equivalent. That's absolutely insane numbers, and many, many things would change, and quite quickly.
If a meeting was called of 10 big tech/manufacturing companies and they were each told they'd be hansomely rewarded if they managed to make large volumes of a missile design, and that all IP/Patent laws would be suspended for the purpose, I guarantee you that there would be massive stockpiles within the month.