My first thoughts are that, if the players are "build" focused and into system mastery, and they expected balanced encounters, then results like those they experienced - not trouncing the encounter - are seen as losing. They got an average grade in class; they want straight As.
If that's the case, I'd present them with a situation where they can choose their own level of challenge. Three dungeons (or adventure locations), one easy, one average, one hard. Then they can play around with their resources and tinker until they are trouncing the hard adventure locations. At which point you can scale up the difficulty.
I'm thinking that with players like these, they would choose the easy dungeon, and wait level up before taking medium dungeon, and so on.
Few sane people voluntarily take on challenges that they KNOW they are barely capable of surviving.
Instead, they tackle things they THINK they can handle, with some difficulty. Nobody climbs a mountain they can't handle and are pretty sure they'll die if they try. Even the beaches of Normandy were won by men who had reasonable confidence that the landing force would win the day, even if some individuals wouldn't make it. Nobody would have gotten off the landing craft had failure been obvious.
So in life, and in fiction, people face greater adversity not by choice, but by mistake, accident or surprise. It is then that people rise to the occasion. The heroic successes being where the person uses the limited resources they have to creatively come to success.
I don't think my view on "why some adventures are hard" negates the idea that some players want to take the easy path. The world is predominantly populated by folks who do what they're good at and avoid what they're bad at.
So if these players play like members of the Dungeon Cleaners Local 447 union, then they're going to balk at entering a level 6 dungeon area when they're only rated for a level 5 dungeon. they don't work on jobs above their pay grade.