Actually, fixed-width fonts arose with the typewriter, which is how these sorts of differences have had time to get established. Variable-width fonts basically didn't exist except in professionally published documents before computers and word processing.
You're not giving enough credit to the printing press. Consider newspapers and books (even the Bible, which practically everybody read), which (I imagine) were printed with variable-width fonts for centuries, right from "the beginning" (i.e. Gutenburg).
Right--but newspapers and books are "professionally published documents." Latterly, there were variable-width fonts in typewriters but that was relatively late in the history of that instrument.
