Because the skill challenge structure has flattened the task from, as an example, "get past the compounds outer defenses" to "get 3 successes".
In the first case, I don't have the skill challenge structure, so I worry--will Hunt make a lot of noise? Will it draw more guards? Do the guards have eyes on each other such that one going down will create an alarm? Maybe there is a nice way to sneak in?
In the second case, I say "Hmm, 3 successes needed. My Hunt is good. Hey GM, Hunting that guard is relevant right? Let's use that to get a success". On a success, I suspect the fiction of the world will be established such that my action was beneficial--none of the above concerns matter.
See, I don't understand this supposed 'flattening'. Lets say the SC you are suggesting is one that we ran for some characters. I would call it a CL1 SC, probably, as it is just some fairly small part of a bigger operation, so I don't want to focus on it too much, it will take 4-6 checks to pass. Now, in a non-SC kind of a scenario, what would this kind of thing entail? It would presumably require climbing, probably stealth, and perhaps perception. I could see Dungeoneering as well, considering it more of a general knowledge of construction and such. So, either way, the PCs approach the outer defenses, looking out for guards (Perception), then sneak up to the wall (Stealth), and then climb it (Athletics in 4e). That's 3 checks right there, with 3 different skills. Maybe at this point Dungeoneering gets invoked to determine the fastest way down, like do we go left or right to find a stairway/ladder/whatever? We're at 4 checks now, so if the PCs have succeeded in all of them, the GM basically says "hey, you did it, you climb down uneventfully and find yourselves in the shadows." Alternatively guard dogs are sniffing around and the PCs picked the wrong way to go to find the stairs, so they have to clout a soldier before he raises an alarm, and manage his dog (Nature).
How hard was that? Does that appear forced in any way? Where are the perverse incentives you find so damning? At what point would a player have an opportunity to spam whatever, Arcana, or Insight, etc.? (any of the other unmentioned skills, the list is almost identical to 5e). At each juncture the players understood what the stakes were, both in fictional and mechanical terms, and acted in a logical fashion. 99.9% of SCs are like this.