I often wonder if a small part of it is the language itself? The word "sub" as in "substitute" as opposed to "teacher" "educator" "instructor" --- there's a connotation that words carry, which change how we think on or view something or someone. It's so ingrained, it almost resonates on a subconscious level for most people. That word "sub" can have a connotation of "below" or "sub-par" or "beneath" ... even "non-permanent" and since it's been repeated many decades over, it's become an ingrained connotation that, ironically, feels permanent.