I like the perspective from jumping into difficult terrain, but you sidestepped the larger question: why are rules system defined crafting etc set DCs generally considered more appropriate than random jumping DCs?
The crafting DCs exist for a different purpose. People were having trouble knowing what they could use tools to make or how they could be used in play. This was added in response to fan feedback and requests.
You'll also notice those DCs are suggested DCs, not hard rules. The DM is perfectly free to ignore those depending on the situation.
Hard DCs for skills are difficult because of the infinite variables.
For example, how hard should it be to jump 1 foot further?
What's the DC to jump an extra foot when leaping from a solid surface to a platform slightly lower with lots of rough rocks as handholds and the wind blowing hard from behind?
How about a jump of the same distance on loose soil against the wind?
What if you're running on a rotting pier covered in water with the waves pounding against the dock and the target being a rocking ship?
Plus, there's a general philosophy in the game that if the DM needs to stop and flip through a rulebook to find out how to calculate a DC the ruleset has failed. The DM is empowered to just make that up, deciding what feels dramatic and appropriate for that session at that moment.
It's the advantage of every single D&D game being refereed by a thinking, sentient being who is largely innately aware of the relevant variables at play and how much of an impact they should have.