Would you imagine if developers added ZKPs or other privacy technique to Bitcoin? That would create a lot of controversy, possibly resulting in mass delistings from mainstream crypto exchanges. With institutional investors getting in the game (Blackrock, Fidelity), it's best to keep Bitcoin as is.
Enforcing every coin to go private would surely shake the investors as regulators would be completely hostile to it. That's what I like with bitcoin. Privacy is not mandatory on a protocol level.
just look at how they treat invested fiat(capgains) different to employment income or inheritance..
How the state treats you financially based on your economic activities is nowhere related to fungibility. Taxation on inheritance being different than in your monthly income is nowhere related to cash being fungible.