Economists argue markets are efficient because of the incentive structure they place on actors. There is a plethora of writing about government inefficiencies due to not having these. It the most distilled form: they are insulated from both positive and negative consequences of behaving efficiently.

Due to this, government entities have a unique set of traits, but efficiency tends to not be one of them. But who cares? Efficiency isn't always the goal. When it comes to making laws, efficiency isn't the goal, long term values are more important.

The post office was created with full knowledge that it would be a net loss, but it was felt to be institutionally important to have universal access to communication across the nation. Now that this is an antiquated method of communication, and there is a market for the remaining services, it is mostly an inefficient make-work program for a well organized voting block.

Now you I can imagine all the distopian problems of a private military, but I'm going to guess efficiency is not one of those (The US Gov believe Blackwater saves them money, btw).