user65819: Bitcoin is not fungible...so why haven't the Bitcoin core devs upgraded the protocol to make it private by default?

user41313: 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.

user63872: Bitcoins themselves are fungible, but the transaction graph is not. This is a small but important nuance: with properly crafted transactions, bitcoin users can gain privacy, without needing to implement more complicated cryptographic constructions.
This is currently an ongoing area of research, for more information you could look into techniques such as coinjoin, payjoin, taproot, the lightning network, etc. Some of these techniques involve obfuscating the chain of ownership through the transaction graph, others obfuscate the nature of the transaction itself, others move payments to a higher level protocol that doesn't leave a footprint on-chain in the first place.
As ieatpizza mentioned, large changes to the protocol can be difficult to implement, but they also bring new risks, as the network consensus will end up depending on new, as of yet untested-in-the-wild code.
In addition to that, the devs cannot just unilaterally decide to change the consensus rules: a majority of users would have to be on board, as would bitcoin businesses and infrastructure providers. It isn't impossible, but it is a very big undertaking.

65819: <p>Bitcoin is not fungible...so why haven't the Bitcoin core devs upgraded the protocol to make it private by default?</p>
 41313: <p>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.</p>
<p>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.</p>
 63872: <p>Bitcoins themselves are fungible, but the transaction graph is not. This is a small but important nuance: with properly crafted transactions, bitcoin users can gain privacy, without needing to implement more complicated cryptographic constructions.</p>
<p>This is currently an ongoing area of research, for more information you could look into techniques such as coinjoin, payjoin, taproot, the lightning network, etc. Some of these techniques involve obfuscating the chain of ownership through the transaction graph, others obfuscate the nature of the transaction itself, others move payments to a higher level protocol that doesn't leave a footprint on-chain in the first place.</p>
<p>As ieatpizza mentioned, large changes to the protocol can be difficult to implement, but they also bring new risks, as the network consensus will end up depending on new, as of yet untested-in-the-wild code.</p>
<p>In addition to that, the devs cannot just unilaterally decide to change the consensus rules: a majority of users would have to be on board, as would bitcoin businesses and infrastructure providers. It isn't impossible, but it is a very big undertaking.</p>
