We who? Do you really want to have every financial move you make available to be scrutinized by everyone?
Mixing does not imply money laundering. In theory in the US you are innocent until proven guilty and exercising your rights to privacy are an important part of preserving freedom. Just because you have nothing to hide legally does not mean your bank account, sexual preferences and the like should be open to anyone else.
Suppose everyone routinely mixes every coin they receive. A haven for money-launderers? Not if we don't want it to be...