[quote=@nerminator]
and space ships obviously cost a lot of time and money to build (unless they don't.) which means it would be stupid to hire fresh-out-space-academy people which could seriously jeopardize the ship
[/quote]

I'm not a GM, but there is a semi-obvious answer to this. Without no fresh people, there would be no veterans, and therefore no military. Without a freshie being on a ship, they do not gain experience and do not turn into a veteran.