spells that you would never want.

[Zanos]

Yeah, because a single CR = party level enemy is not a tough or interesting fight most of the time. Most DMs run harder fights, which does decrease blasting value, but keeps save-or-die value about the same, because it bypasses everything but saves, and saves don't scale as hard as HP.

Their value isn't the same because saves do scale rapidly with CR, but their value is less impacted because they don't have to overcome a second defense in the form of HP. A creature that fails it's save vs. finger of death or slow or sleep is just as affected regardless of it's challenge rating, while the same isn't true for damage.

That said you often have to deal with immunities for save or X spells. While many monsters will have immunity or resistance to various energy types it's pretty easy to compensate for that, and you'll probably never run into an enemy that's just immune to hit point damage. Can't say the same for slay living, or even stinking cloud.