Because it can be stupidly easily controlled by the government. If there’s a pandemic and they don’t want you going more than five miles from your house, they can literally turn off your money if you are more than five miles away from home.

Did you eat too much meat this week? Your money gets turned off because you used too much carbon.

Did you take part in a protest the government disapproves of? Good luck eating for the next month with no money to pay for your groceries. Have fun starving.