Why?
We've all had bad experiences with a half dozen or so bureaucrats but effective governance is based on continuity and that is what govvies provide.
There are bad apples in the bureau of weights and measure just like there are bad apples building airplanes at Boeing (
http://en.wikipedia.org/wiki/Enumclaw_horse_sex_case) , but working for a civilian company that provides contract services to the US Government is different than working for the government.
Under your system, every mayor, governor and president could can all employees who they didn't personally like and plug in "yes men."
Again, I've worked with a lot of government contractors and they all knew they could be gone the next day for no real reason.