Government is just people. The market is just people. To take an example, let's look at Medicare. A lot of progressives want everyone to have the option of buying into medicare. If Medicare is run superiorly to the free market, can't the people who run Medicare just set up a private business and outcompete existing private medical care/insurance providers. Does something magical happen to people when they become part of the government? If people are greedy and selfish, doesn't that mean that the people who make up government are also greedy and selfish? The only counter to this argument is that the people who enter government are somehow morally superior to those in the private sector. I am confident that history has shown that this is not the case.
Restated. If government provision of a service is superior to market provision, why can't the people in government just outcompete their private sector counterparts without the force of government behind them?