CBDCs enable all sorts of horrible, totalitarian things.
They allow governments to track and control every penny you earn, save, and spend. They are a powerful tool for politicians to confiscate and redistribute wealth as they see fit.
CBDCs will make it possible for central banks to impose deeply negative interest rates, which are really just a euphemism for a tax on saving money.
Governments could program CBDCs to have an expiration date—like some airline frequent flyer miles—forcing people to spend them, for example, before the end of the month when they'd become worthless.
CBDCs will enable devious social engineering by allowing governments to punish and reward people in ways they previously couldn't.