It's impossible to explain why no items are generated on stair levels in ID from the game-lore point of view (as long as gut doesn't join the discussion).
Instead, why not differentiate specific spellbooks' DL?
Rather than DL1, make all ball spellbooks have a DL 5 or 6, teleport a DL 8-10, wish is already rare enough, create item/mystic shovel/death ray could have even higher DL to reflect the power of respective spells they grant.
This would effectively remove the most powerful spells from early game availability, but still leave fire/frost/lightning/magic bolts around for picking at an early level, since the wizard character already has one of those from the start.
I never understood why all spellbooks have the same DL despite such obvious differences in usefulness.
It the same situation as with potions - potions of water have a DL of 3 or 4, while PoGA - DL1. How much sense does that make? Should be the other way around. Why vials left after using potions of water are discarded? It should be possible to refill them using water from an underground river or even lake... but I'm straying a bit away from the subject.
The fact that wizards are slightly overpowered compared to other classes has nothing to do with having several mandatory spells at an early stage of the game.
The power of wizards comes later in spell cost reductions and ability to read spellbooks effectively. Finding those earlier does not mean you can breeze through the rest of the game, not more than getting emerald dagger in UD vault playing an assassin. It doesn't mean you can't die. With careful playing every c/r combination can win, with enough carelessness, even the best PCs die.
As for ID exploration, it takes a lot of real life time compared to in-game results. Believe me, I explore ID often because stair hopping is boring and tedious, but it is the tradeoff of getting useful items.
Next thing I hear will be RFE regarding vaults - remove the drops because they are too easy. Then remove tension rooms because those can be abused too. And so on.
If you don't like ID stairhopping, simply don't do it. It doesn't give a boost big enough to overall character performance to justify the removal of item generation in stair rooms and chars made this way don't exactly skip half of the game since they are so powerful after visiting ID.