The randomness of receiving just the right item is what makes OP Runs. The whole point of playing is to hopefully get an OP run; 90% end in failure before Floor 11/Endless.
An OP run means winning all encounters by repeating the same combo over and over, like stacking Haste and block so you never take damage. There's no fun in OP builds.
Recovering all mana charges at the end of a fight means you can use wands carelessly. That's OP. Limiting the recovery to 1 or 2 charges would incentivize managing their use strategically.