It really isn't years worth of useful knowledge, the military is capable of giving recruits the knowledge they need much more quickly, especially given the much more disciplined/controlled environment involved.
In a year you can teach kids who are a little behind enough to get them up to the min high school will teach them, but certainly not the most that high school will teach them if they want it.
Of course this overlooks what the minimum bar for a college is. At my college the lowest math they offered that wasn't considered remedial was calculus. That's a far cry from many community colleges where the lowest is alegebra. You definitely should have learned alegebra in high school and not knowing it definitely extends your timeline on certain disciplines.