One could say the same thing about Harry Truman (a mere laison officer in the artillery during WWI) or John F. Kennedy (a mere PT boat commander during WWII), and I could go on and on about junior officers and enlisted men who returned from overseas to serve this country well in new capacities.
There is a good argument that certain military experience is very good preparation for being President; e.g., George Washington, U.S. Grant and Dwight Eisenhower. But, for most people who have served honorably, what military experience demonstrates is patriotism and physical courage. I cannot imagine that Bob Dole or John Glenn would ever sell us out.
With regard to some high-ranking officers, like Wesley Clark, it doesn’t appear that they are able to translate their leadership and command style to the civilian world. George Marshall or Colin Powell, on the other hand, moved easily into new positions of responsibility. As to why some can make the change and others can’t, I’d say it is because many military leaders use the authoritarian style, which doesn’t work so well in the civilian world.
With regard to John McCain, being a naval aviator, he was kind of on the wild side as a young man, which may or may not be the right kind of “experience” for a leader of a free society. His “experience” in the Hanoi Hilton, as he develops in his autobiography, deepened him, and his faith in God and his devotion to this country.
Just heard on FoxNews that when Clark was running for President four years ago, Clark had to ask an aide to clarify to him (Clark) what his (Clarks)stance on the Iraq War was. LOL!