In the military there are two separate classes, officers and enlisted. Officers had at least a bachelor's degree and the military is a much different experience for them. They come in day as leaders, are paid significantly more, and have more opportunities to make lifelong connections with powerful people.

Enlisted are treated like blue collar workers, the grunts. Just in their day to day jobs enlisted people are going to have more wear and tear on their bodies. Take battle experience out of it, just as is, officers have a much better path forward.

Fraternization is illegal between officers and enlisted which basically makes it two segregated classes.

Enlisted folks also tend to come from lower income families, so while they are giving more opportunities than staying in their home town it can still be difficult for them to advance much higher.

In short, you have a large mixture of people from different ways of life with different education levels. Some take advantage of stuff like the GI Bill or other opportunities,. others don't. Also some give their bodies to the extent that they can't really function at the same levels on the outside.