The military attracts plenty of geniuses just like any other large organization. Simply by not admitting the least capable the Army ends up significantly higher IQ than the average high school. Meanwhile 39% of 18- to 24-year-olds and 65.9 of those who graduate high school were enrolled in college, so it's not all that selective.

PS: Yes, this means that large numbers of smart people don't think the way you do.