... because it already exists.
For most people, your money is in a BANK. The bank has a ledger of every single transaction you've made with your credit/debit card (because who uses cash anymore?). If they want to, not only can they find out exactly what you've purchased at a particular vendor, but they can also find out what you look like since most businesses have cameras.
Sure, they could make an even more trackable digital currency, but the fact of the matter is that what most people fear with regards to a CBDC already exists. You'd be better off using Bitcoin or cash and keeping as little of your cash out of the bank as possible.