If cryptocurrencies are private enough to obfuscate transactions to the point that authorities can’t monitor illegal financial activity, how can the system also

If cryptocurrencies are private enough to obfuscate transactions to the point that authorities can’t monitor illegal financial activity, how can the system also be transparent enough to effectively tax users? Isn’t taxation a centralized financial construct, in that a central authority (government) assesses taxes? If the coins built today become the mainstays of crypto transactions and don’t have a concept of a transfer tax, how will one ever be added to the system? The entire system seems to be built from the perspective that it’s “our playground, our rules”, but why governments ever allow that?