I’ve always wondered if NASAs obsession with “perfect astronauts” really had enough merit to warrant itself. My personal conspiracy theory was it was more about not losing face to the USSR then it was any practical need.
They will take 10 years teaching astronauts about everything from orbital dynamics to spacewalking to Siberian survival.
It's literally millions of dollars spent on one person, so you need to maximize the probability of success. It's why they go for perfectly healthy people, both physically and mentally. Can't have someone quickly out of breath, or someone who breaks down in a stressful situation.