Where does this expectation come from? Previous editions? The tone from previous, more mundane adventures that occur before the high-level things? Or from the way the book is written? Personally, from the way the book describes high-level adventures, I can understand why people expect all characters are assigned powers that seem to scale appropriately to the danger at-hand.
I think all sorts of places.
- Fantasy literature being one (Conan is the hero of the story, not a supporting character, and does so without any 'superhero' abilities).
- Also, action movies where Wesley or John McClane or whomever Ahnold is playing effect the situation without abilities which defy a semblance of reality (if not probability).
- Also the early versions of the game (or people's memory thereof) where high level fighters were powerful (it should be mentioned that this had no small part to do with magic users still being tissue-paper flimsy at high levels and the fighters having a bunch of magic gear which let them fly or decapitate enemies or sometimes even shoot fireballs out of their swords 3/day or similar).
- Or even Chainmail itself where a Superhero (8th level fighter, in effect) could waltz through a battlefield cutting a deadly swath while being immune to dragon fear and the most spell resistant normal army unit.
- Other game systems (particularly the point-buy ones) where, for the resources it takes to be good at spellcasting in even a couple of key areas, you can also make a polymat warrior-skills expert who can actually do many many things and contribute meaningfully at least as much as the caster (even though the caster still has things the non-caster can't do).
I'll add my own Subsection:
Whenever there is an out-of-combat scenario, there is a magical answer that requires a caster, uses a spell slot, but works (often completely, and with broad application). There is also (sometimes) a non-magical answer that is limited (usually to Guy at Gym levels), requires a roll, and has the option of being detrimental if you fail to make the roll.
Counter(#1): there aren't that many 'fail catastrophically' rules in the book, that's mostly DMs doing that themselves.
Counter-counter: yes, but DMs keep doing it, and a more rigorous OOC resolution model (or moving spellcasting into the same model) would increase parity
Counter(#2): those spells take up memorization room and spell slots. If that's not a constraint on your caster, your DM needs to make more (and varied) encounters per rest scenario
Counter-counter: we're back to blame distributing instead of building a framework that solves the issue. Given that we're looking for guidelines or DM's aids or alternate rules rather than taking anything from the people who already have a system that works for them, this seems unhelpful for no reason.
Counter (3#): The open-ness and vagueness of the non-magical resolution system is as much benefit as limitation. The skilled non-magical character can do anything they can convince their DM the skill allows, while the spell descriptions are absolute limits on what the magic can do.
Counter-counter: like #1, DMs just seem not to give that greater flexibility (or give casters as much leeway). If you find the DM that lets your fighter use athletics to turn (for example) a medieval construction site into a donkey-kong like obstacle course for your enemies, this works out for you. For the person whose DM makes you roll to quietly close a box-lid, they are SOL. Again, just asking for in-book guidance and DM aids takes nothing away from those who don't see a problem.