You may be overestimating the number of successful recruits. I took a test for an ATC job application back in the late 70s. It included listening to increasingly long lists of words and numbers and writing them down after a short delay. I'm pretty sure I didn't do that well, but they kept my name on a list of possible hires for a year or so until they notified me that I would have to respond in order to keep my name on the list. I had gotten a different job in the meantime, so I declined. I don't think they were hiring very many controllers at the time, but if I had kept my name on the list I might have been hired when Reagan fired all the striking air traffic controllers. Anyway, that test would be pretty hard for most people.