Difficulty is measured relative to the world as a whole, and the (global) average modifier to any given check is between +0 and +1. Moreover, in case anyone has forgotten, a natural 20 is not a success unless you're talking about attack rolls or saving throws.Exactly my point. Why is 19 the number chosen? If the PC's capabilities are not a factor then why aren't difficult tasks actually more difficult? Why is being caught in a blizzard just hard enough to be challenging to the PC's but not overwhelming?
If a Hard DC was anything higher than 19, then it wouldn't be hard for most people - it would be impossible.
Which jives well with our observed reality. In the real world, normal people sometimes beat the odds and survive terrible circumstances.