In high level games, many opponents have enough HD that all their saves are boosted above the wizard's spell DCs. In both AD&D and 3e, many high level combats devolve to where only a 1 fails a save, or maybe 1 to 5 in some cases. It's very rare for a monster or NPC to have a completely soft save except through a combination of bad luck and poor planning.
I'm not sure "rare" is exactly correct depending on what we mean by "completely soft". [I'm quite open to being corrected on this point]
Imagine an elite array wizard (Int 15) at level 16 with a +2 from a tome and a +6 from a stat adder (much as I despite them). It's reasonable that her Int will be: 15 + 4 (level) + 2 (tome) + 6 (headband of intellect) for a 27 INT.
That gives her a +8 add to spells so her 6th level spells have a DC 24 to save (that being 2 levels below peak, let's not assume the perfect spell is available at 8th but rather that something useful is in the top 3 spell levels).
From the SRD;
1) Titan (CR 21) Saves: Fort +26, Ref +13, Will +21
Needs a 2+ to save Fort, an 11+ to save reflex and a 3+ to save will (and is 5 levels above the wizard (ut is a good example if we shifted to a level 20 wizard)
2) Nightcrawler (CR 18) Saves: Fort +12, Ref +10, Will +23
Might as well not bother with a will save based spell but that distintegrate needs the beastie to roll a 12+ to save and it's even worse with a delayed blast fireball.
3) Marlith (CR 17) Saves: Fort +19, Ref +14, Will +14
Fort saves against a 6th level spell on a 5+, requires a 10+ to save versus a 6th level spell if it targets will or reflex.
These are all critters in the same range as a high level wizard. Even dragons tend to lag in reflex saves. These weak saves aren't completely soft but the 13 point save gaps in the Nightcrawler or Titan are not unimportant.
The Marilith is more balanced but save targeting can shift a "save or bad thing happens" from 20% chance (disintegrate) to 45% chance (Repulsion to keep those swords away). Some creatures are very balanced (Balor) and I see that as a major advantage for these monsters.
I also did not add spell focus and greater spell focus (the key spell might not be in the correct school) but these can also make the weak save of the Titan (for example) really show up (as in it has little hope of saving). A DC 28 level 8 spell in a school with focus and a reflex save is saved on a 15+ for the Titan and 18+ for the Nightcrawler. And a pearl of power (core item) can handle that agonizing 19 for the first save roll should it happen . . .
Sometimes these things are subtle but I have a long experience with players just opening up with the (frequently prepared as it is often useful) disintegrate spell when undead appear after level 14 or so . . .
I agree, ahead of time, that this maybe different in fully supplemented and homebrewed 3.5E where the DM is adapting the creatures to the (far tougher) late 3.5 character builds.