Bitcoin does not and never did have well-defined requirements or constraints, so it is kind of pointless to ask why some feature is not present or why the creators of Bitcoin have not worked to improve it in some particular way. None of the definitions of privacy or security defintiions that apply to academic digital cash can be satisfied by Bitcoin, as those definitions all assume the existence of a central bank in the system. Similarly, definitions that are applicable to anonymous messaging (e.g. mix-nets) are useless here unless you want to use Bitcoin as an anonymous messaging system (rather than as a digital cash system).
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Bitcoin does not and never did have well-defined requirements or constraints, so it is kind of pointless to ask why some feature is not present or why the creators of Bitcoin have not worked to improve it in some particular way. None of the definitions of privacy or security defintiions that apply to academic digital cash can be satisfied by Bitcoin, as those definitions all assume the existence of a central bank in the system. Similarly, definitions that are applicable to anonymous messaging (e.g. mix-nets) are useless here unless you want to use Bitcoin as an anonymous messaging system (rather than as a digital cash system).