So I'll be happy to discuss that when y'all admit I'm probably right about the actual answer to this puzzle, that being that Gary Gygax was attempting to emulate a very specific and bizarre type of spellcasting, that of the Dying Earth books, which doesn't feature people just fumbling spells like a putz as a trope (unlike many other takes on spellcasting). My suspicion is that the thinking is the way those spells work means you can't "naturally" screw up the movements - it takes an additional factor (armour restricting movements, other people interfering, getting hurt during the cast, and so on).
Either that or that it's so unlikely with Vancian casting as to not be worth consideration. A similar approach could be found in the old 3.XE Coup de Grace rules where you auto-hit and auto-crit even though obviously IRL, you could screw up such a manuever - it would just be probably less than 5% chance thus not really meriting rolling.
You said before we discuss balance, we need to answer the question of why this is, to answer the puzzle. I believe I have answered it correctly, but literally no-one, including you, has responded on that. AFAICT all the evidence supports my contention. Genuinely I just want someone to say "Okay, you're probably right re: the puzzle, now how to we make spell failure a useful mechanic?"
(I do have some ideas as to how to make a single-roll mechanism work with the spells you mentioned btw, I'm not just being a tease!)