Fuckton of things, ranging from health & medical issues like obesity and all manner of conditions to legal status & drug use, not to mention the actual physical fitness standards that have to be met before entry. I can look for all the metrics if you’d like, the 74-80% number is from an official pentagon report on national military recruitment and readiness
The Japanese were preparing for an invasion of their homeland, they were also an utterly fanatical society that drilled into every citizen the fact that their life existed to serve and die for the emperor. Little bit of a different scenario, considering it’s not even remotely possible to invade the U.S., and the wars we’re preparing for don’t require high amounts of ground personnel, but rather innovation in technology. Especially in the Indo-Pacific