FormerlyHemlock
Hero
Check out the MM. Are there a lot of CR 20 critters throwing down DC 10 saves? Or CR 11s, for that matter. No.
Bounded Accuracy says you should not assume anyone will pass a saving throw initiated by a CR 20 creature. If they do, great, but you shouldn't assume so.
Bounded Accuracy says you don't need to use CR 20 creatures. If you want a saving throw everyone will probably pass, build a 20th level challenge out of lower-CR creatures. How does that 20th level party feel about a Hard encounter consisting of a young white dragon and 80 magma mephits?
That's Bounded Accuracy for you.
If you are a high level character, you're going to face attacks with a high bonus, and you're going to face saves with a high DC. Not every time, and you can try to avoid them as hard as you like, but when the former happens, you can quite easily have a decent AC, while when the latter happens, covering all your saving-throw bases is impractical.
For those who play without feats or multiclassing, those are equivalently difficult tasks. A 20th level bard (AC 12-15ish) is going to get hit 95% of the time by an adult red dragon with its +14 to hit; he's going to fail his DC 19 (Wisdom) saving throw against the dragon's frightful presence 90%ish percent of the time too. He can mitigate his AC problems or his problems with fear by staying out of range; he can further mitigate his AC problems by hiding or with Spell Mastery of spells like Shield; he can mitigate his problems with fear by pre-casting Heroism or by hanging close to the Paladin or simply by innoculating himself with a prior encounter plus some nice cool-down time afterward.
There's always feats and multiclassing on top of that, but the fact that AC is easier to boost via multiclassing than saves are is the fault of the multiclassing rules, not the saving throw rules. Probably only powergamers really witness that asymmetry--I know that when I first started playing 5E, it did not occur to me to solve my Enchanter's fragility in melee by simply multiclassing a single level of Life Cleric.
Actually, it's kinda nonsensical. You spend your whole (probably short) life adventuring, and never get better at most adventuring tasks? Just the ones you were 'proficient' in when you started?
Just the ones you practice. If your wizard is really trying to become better at dodging dragon fire, it will manifest itself as +2 Dex or Resilient (Dex) at some point. If instead he is focusing all his attention on the arts of the mind (+2 Int, Observant), who can be surprised that he didn't get better at dodging dragon fire? At least he still got better at surviving dragon fire, thanks to his increased HP.