Where do governments get their money from? Taxes on economic activity. The more economic activity the government performs itself, the less opportunity there is to raise tax revenue.
Take this through to its logical conclusion and you have the government owning farms, making food, making its own steel, building its own cars, etc. with a corresponding loss of revenue-raising activity in the real economy.
Take this through to its logical conclusion and you have the government owning farms, making food, making its own steel, building its own cars, etc. with a corresponding loss of revenue-raising activity in the real economy.