It's about people being able to trust their currency. Let me explain: suppose that your government was creating a huge amount of money out of nothing. It would result in a massive inflation and, with the value of money being completely unstable, people would resort to alternative ways of paying for thing: a more stable foreign currency, gold, crypto currencies, or even barter. You can see it happening in countries with a massive inflation (Zimbabwe, or Argentina in the early 2000's are the examples that are on top of my mind). So, this is a scenario the governments have to avoid at all cost, and in theory they should create just enough money so the inflation matches the growth of the country.
However, the growth of the country is very hard to measure (directly taking the growth of the GDP would not necessarily match it), so there is no consensus on what the inflation should be exactly, which gives the authority in charge of creating money some leeway on exactly how much money to create. If this authority was the government, it would be tempting to have the inflation be slightly higher than the country's growth, in order to "trick" people into believing that they got slightly richer and have them spend more. And this could work for some time, but at some point people would become wary of the government and as a result would spend less, slowing down the economy. So it becomes a game (in the sense of game theory) between the government and the consumers, and the solution is to have an independent entity in charge of money creation, so people can know with more certainty that the "right" amount of money is being created.