They do admittedly. But for instance the guy who drives leapord 2's for ten years isn't a qualified Fighter pilot. And isn't likely to leave the army suddenly to do so. People do change MOS. My brother did as well. But they are usually within the same service branch and the Super drastic changes of MOS aren't as often as people think. They happen sure. But most people stay relatively within their little service Sub branches.

And now that you said I am going to be missing out on a lot of plot and I don't really feel inspired to make a pilot anymore.