Pedantic
Legend
Just touching on this particular point, 5e is a follow-up and expansion to the 4e rejection of codified task DCs that started with skill challenges. We were moving solidly in the direction of not making task difficulty setting (and the attendant on the fly game design) a DM responsibility in the 3.x era and could have gone further (cleaning up the trap rules, for example).This is similar to the questions around (i) GMs declaring uncertainty in 5e as well as (ii) mediating DCs via landing on (iia) genre emulation vs (iib) physics simulation and/or (iiib) transparently establishing (or not...which is typically the case) the Medium DC benchmark (is it "Average NPC Joe" vs "Expert NPC" vs "Hero of x Tier" vs "Hero of y Tier" vs "Hero of z Tier") for DC setting in 5e. It isn't enough to say "trust the GM...they will do this competently." That GM process needs to either be transparent and codified or it needs to be absolutely telegraphed (like via meta-conversation) and inferable for players to be able to reliably do their "gameplay necessaries" in orienting to the situation and then executing a sequence of moves which facilitate a desired gamestate.
More to the point though, this is not an inevitable feature. DMs could be bound by strict task rules they didn't create and that are known to the players. Creating and maintaining a setting does not necessarily have to include determining task difficulty. Frankly, I think gameplay would be much better in general if more play focused on players deploying known moves against unknown situations.
The current model, as far as I can tell, seems to exist to put upper limits on player power; by not specifying the precise climbing rules, you can always put a harder climbing obstacle in, instead of having to contend with a player who can't fail to climb things. That and because it hides any real rules complexity behind DM on the fly decision making, streamlining the player experience (though I'd argue to the detriment of that experience as gameplay).
There's more than 2 gameplay loops in tension here.