What happens when it becomes so good that it's the only option? There's no need for it to be profitable, so it can inevitably undercut private sector competitors. Do you want to live in a country where the central bank controls and monitors your ability to transact with other people?

Just last year, the Canadian government weaponized "debanking" in an effort to quell peaceful protests against vaccination mandates. They restricted the bank accounts of people who peacefully disagreed with government policy stances. And that was just with the status quo of deputizing private banks to enforce the sanctions against the individual citizens - imagine how much more systemized and effective it would be as a coercion measure if the bank accounts were directly managed and controlled by the government.