Neither olive oil nor vinnegar need to be refrigerated; yet when mixed (as
in home-made salad dressing), the recipes all ask for it to be refrigerated
(where it congeals to a gooey mess).

What is different about olive oil + vinnegar + a few spices & salt from a
microbial culture standpoint?

Why must I refrigerate a mix of that which doesn't itself need to be
refrigerated?