They can be supplied with potions elswhere. Not everything has to come from the person itself. It's mostly about the administration.
Classes are far from self-sufficient. As someone said, we had quite a time when people needed reagents from people. Rogues had to buy poisons from an apothecary (Ooh Rogue's can't make their own. Wait, poisons are chemicals, this is alchemical kind of stuff, how dare they, remove this now! How dare rogues try to take this away from alchemists!), Druids needed a reagent to cast Rebirth, Mages needed reagents to cast Portals, Shamans needed ankhs to reincarnate, and Hunters needed to purchase or go engineer and make their own ammo.