Latin Abbreviations: "I.E."

       "I.e." comes from two Latin words id est, which simply mean "that is."  Why do people, when reading aloud (and I guess to themselves silently too) almost universally read this expression as it appears . . . "i.e."? 

       Most people know what the term "means," at least approximately.  And I don't expect you to say "id est" (though you could since you now know what "i.e." stands for, and you probably say "et cetera" all the time), but why wouldn't you now say "that is," a direct and easy translation into English, which gives the exact meaning of the term?! 

       In fact, why don't people write "that is" instead of "i.e."?

       We get caught up sometimes in unspoken hierarchies of written language over oral, of classical over modern.  I say "Know it, tame it, turn it into English."