Parallelization would make high level casters utterly broken. One of my favorite spells, because I made it for 3.5 edition, and I haven’t had a DM criticize the spell, is Unfailing Missiles.

If I could cast three of them in parallel, I would be able to hit up to 9 targets that are no more than 20° arcs from the last target, each with a missile that does 17d (6+1) points of Force/Sonic Damage split 50/50. Or I can use all of those missile on a single target. If I maximize the spell with 3 12th level spell slots, that’s 1,071 damage or 119 damage per missile. There’s no save, please refer to the name of the spell. This is basically a 9th level Magic Missile. If I see you, I wollop you.

I would then need to hide for 12 seconds. Not particularly difficult if one has a Contingencied Improved Invisibility, or a Contingencied Screen.