Because those pilots theoretically move "up" to being staff officers, and then a few of them reach the highest levels of command, with the benefit of a diverse background.
It turns out that being Erich Hartmann, Maverick, or the Red Baron doesn't really translate well into being a good executive leader, and while western militaries want officers who have experience "at the sharp end", the also require leaders who understand how a headquarters works, logistics, politics, etc.
It turns out that being Erich Hartmann, Maverick, or the Red Baron doesn't really translate well into being a good executive leader, and while western militaries want officers who have experience "at the sharp end", the also require leaders who understand how a headquarters works, logistics, politics, etc.