The lack of a stockpile is due to limited shelf stability and long-term bacterial growth.

It's unfortunate Reason doesn't expand on the other side of this issue: the formula industry lobbied the FDA to reduce bacterial testing frequency (and inspections overall), with an emphasis on Cronobacter risks, arguing that the FDA "overestimat[ed] the expected annual incidence of Cronobacter infection". [1]

[1] https://theintercept.com/2022/05/13/baby-formula-shortage-ab...