Look, by "won't reteach" you give it away. Those who are not rusty in linear algebra will tend to be recent grads, younger people who are probably not looking for a boring defense job with no remote somewhere in Alabama. Those jobs appeal to people who may have had enough time to forget how to implement a SVD without googling.
Alternatively the developers who do remember and use linear algebra many years into their career or even more advanced math/numerical methods and are also interested in using it are in higher demand than ever before due to the explosion of ML/AI, and thus can command a much higher salary.
The simple answer is always they simply don't pay enough to attract the people with this skillset. If they paid as much as Meta (who they used as an example) they would certainly have way less issues with hiring.
On the latter point I agree completely, on the former--demand for mid-late career quant types, I want to agree but also think we'll need to wait and see because current AI will lower a lot of bars, and as you acknowledge it's all about money ultimately.
