Forgive me if I am out of line, here, but isn't this the chart in question?
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I agree DC 20 is high, but it isn't "crazy" IMO. IME AC 20 and higher isn't hard to do, most PCs will have that by tier 2 if they are defensive builds at all.
You are correct, only a few creatures have AC 25, and that is the cap for creatures IIRC.
Guidance is for skills, not attacks.
Like attacks, most PCs have skills which they excel at via high ability modifiers as well. So, you are likely to have the same modifier for both:
Attacks: proficiency + ability + magic (item or bless, etc.)
Skills: proficiency + ability + magic (yes, few items but guidance as at-will cantrip...)
But, FWIW, AC 20 is not "common" really until tier 4 encounters (depending on where you draw the line with the CRs, of course), but by then you are typically +10 or +11 (without magic weapons), so, you are still in that "wheel-house" of needing an 8 or 9 and having the 60-65% chance of success IME.
For skills, it is pretty much the same since guidance can be used in lieu of magic items. But if you throw in expertise, it makes even DC 20 at that level nearly an auto-success...
Anyway, DC 20 is
hard, and trying to convince a hostile creature to do as asked should be hard. But even a CHA 16 with prof +3 would only need a 14 or better. Throw in guidance and you are looking at a 12. Have another PC
help in trying to convince them (or gain advantage through other means...) and now that "hard" task has about a 70% chance of success... Again, throw in expertise and it is fairly easy (84%; DC 20 with +11 and advantage).
Finally, why are DCs different from ACs? Well, because ability (skill) checks are not combat. Remember with ability checks, unless there is a meaningful consequence to failure, often you don't even need to bother rolling. Combat is much more "intense" in that fashion--there is always a meaningful consequence to failure. Keeping combat success in that "sweet spot" of 60-65% is fairly easy. Most ability checks range from DC 10-20 anyway, with very few every above that (the same is true for ACs and saving throws). So, I don't see them as being that different really. IME, I would say your typical ability check is probably lower than your typical AC.