What you're doing is comparing 20% (flat number) vs 20% (effective gain in %resist for the last 5%). Basically, you're doing an apple vs orange comparison and forget to consider how it applies in-game.
I'd argue that's what you're doing.
Being able to take 25% more damage (or 20% reduction, however you prefer to phrase it) has *more* of an in-game impact at lower numbers. Why? Well, let's take it to an absurd extreme. Which is better, going from 0% -> 90% or 90% -> 99%? Mathematically both give you 900% more survivability...but you don't *need* that survivability if you're already at 90% reduction. You'll notice a difference, sure, but you'd be in no risk of dying regardless. While that person at 0% reduction really, really sees a massive practical difference.
Or, if you prefer, look at Routine difficulty. Technically speaking, a bunch of Willpower promotes would increase your survivability on Routine (since Armor will reduce all physical damage to 1 anyway)...but you don't need the survivability on Routine in the first place.
Someone who goes from being able to take 4 hits to 5 hits notices a massive difference -- they'll survive situations where that one extra hit (a mere 5 hits total) would have previously killed them. Someone going from 20 to 25 hits may notice a difference, but at that point you can take so many more hits that the increase has less practical value.
In real world terms, it's like going from 20k to 40k a year vs going from one million to two million a year. If you're already making a million dollars a year then the extra money isn't nearly as life changing. Even though both are technically 100% increases.
Furthermore, players with 75% resist will usually also have better strategies, skills and timing, all of which improve considerably your survival too.
I think that's a misleading statement.
If you mean "Pick a person with 30/30/30 at random and then pick a person with 170+/170+/200+ at random" then sure, odds are extremely good that the 170+/170+/200+ will be a better player...but that's solely due to the self-selecting nature of the latter pool while the former pool is polluted with boatloads of people who don't care and don't try.
On the flip side, if you spectated a Nightmare run and saw someone with 30/30/30 and another person with 170+/170+/200+...the odds aren't skewed nearly as badly (though they'd still probably favor the Promotion Lord). Why? Well, among other things, a lot of highly promoted people are used to being able to get hit with so much stuff because they have insane stats. They don't *need* those strategies, skills, or timing to survive. Stuff that would flat out kill the 30/30/30 will only take away like 15-20% of the Promotion Lord's health. And they get used to facerolling through stuff while the 30/30/30 is struggling to stay alive and figure out any tricks they can.