Even if the postal service were privatized, the government would require that private carriers deliver mail to all citizens. In government, the bottom line is not profits but voter satisfaction. Hedge their risks. Business executives who take bad risks normally balance them out with more calculated ones. Regardless, if investors think the company takes unwise risks, they can always pull out their funds.
Government works differently.
For years, Orange County Treasurer Robert Citron received national attention for earning some of the highest returns of any municipal investor. Of course, when his bets on interest rates soured during 1994, Citron lost millions and forced the county into bankruptcy. He lost his job and even landed in jail. And unlike in business, taxpayers couldn't pull out their funds. Certainly public officials can make strategic bets and still survive if they fail.