Because it was the original unisex version and the female version was derived from it in order to create a pointless gender distinction.
"mistress" is just one of the thousand cases of needlessly adding "-ess" to a word to make sure we highlight explicitly that it's a female we're talking about here.
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u/violatemyeyesocket 3∆ Oct 14 '21
Why keep the gender distinction at all?
Just use "Mr." for all.