Why isn't Bitcoin private by default? Bitcoin is not fungible...so why haven't the Bitcoin core devs upgraded the protocol to make it private by default?
They are trying to add new features that would help with privacy! There is an upgrade proposal known as Taproot that would come with features to enable additional transaction privacy in Bitcoin.
Note, however, that major changes like this to Bitcoin are much more difficult than they are in smaller altcoins, simply because a significant amount of infrastructure must be updated, and getting so many different people and organizations to agree to a major update is challenging, although necessary to avoid a fork in the network. Also, there is an effort to keep updates backwards compatible with existing infrastructure, which limits the extent to which major changes like this are possible.