Because we have spellslots instead of Mana / Sorcery Points (or what that Variant rule is called). Everybody who have used the Variant Spellppoint Rules to replace spellslots makes the same experience: Caster always use the highest spells. Instead of 2 or three fireballs, and then shatter and then catapult, it is fireball all the way till they got no more Mana.
Yeah, optional rules that replace rules and increase the difficulty for the players are rarley used. Gritty Realism. Variant encumbrance ...
Optional rules that give players benefits are like always taken (Flanking).
If given a choice, players will always opt to make it easier for themselves, even if it kills the game because it becomes to easy.