The main reason why he doesn't do it is because they're pretty much all personal friends of either himself or his dad. It's a common problem when you surround yourself with incompetent cronies; it'd just be too awkward and unpleasant for him to let them go.
I think this hits it right on the money, and is a clear indicator of why you should
not hire your close personal friends to positions immediately under you.
Of course, Bush's entire term has been about helping people he owes things to, so I suppose I can't be too surprised...