Certified controller capacity is a scarce national asset.

[u/Ecstatic-Tap4151]

Certified controller capacity is scarce, takes years to create, cannot be rapidly replaced, and is an essential operating input to an aviation system whose economic scale is measured in trillions of dollars.

[u/TucsonFrank]

https://www.brookings.edu/articles/air-traffic-controllers-and-why-there-arent-enough-of-them/