I'm with Major Matisek. Mental fitness is a far higher priority.
Unless someone is actually disabled, I can have them physically fit within 6 weeks. Mentally fit takes years of work.
There is a "chronically underperforming" educational system in
some areas, but the majority of schools are just fine. For purposes of the military, we have plenty of sufficiently well-educated people.
The physical thing...yes, you can take someone and get them in better shape in six weeks, but it may not be anywhere close to the kind of shape you need to be in for high intensity military activity, including covering distances at a sharp pace while wearing heavy gear, etc.. You cannot simply build up that kind of strength and muscular endurance overnight, and perhaps more importantly, connective tissues take much longer to strengthen.
There are plenty of people who wash out of boot camp because they are too lacking in core strength/endurance to meet standards, despite being given more than 6 weeks of physical training. It's one thing if we're talking about someone who did have good strength but simply got fat. Underneath the fat is fine. But it's something else when you're talking about young men who never developed sufficient muscle mass in the first place.