Personally, I have a couple of reasons.
1. My first obligation is to my family. It is my job to watch over them, protect them and provide for them. With my skills, I can do that better here than in the military.
2. I prefer building to destroying. A military may well be necessary, but it accomplishes its ends thru destruction -- killing, bombing, etc. I, personally, would rather have a more positive role, and leave the killing to those with the temperament for it.
Actually, I have more reasons than those two, but one of them (at least) would start a flame war, and I don't want to do that. The first reason alone is enough to keep me out.