You don't want to make lootable non-craft herbs too prevalent or they start to undercut the value of becoming a brewer.
Potions have one great advantage over herbs: weight and stackability. 10 potions of the same type take up 2 inventory squares and weigh 1lb, whiile ten herbs of the same type will take up five times more space and three times more weight capacity.
Furthermore, many effects either can't be replicated by potions (Chamomile: Sleep, Wintergreen: Remove Curse) or, when they
do offer an effect that potions repliciate, these potions are already available from a NPC seller (Cure Minor Wounds at Bervis the Beekeper; and both Cure Light Wounds and Antidote potions are available in bulk in Hazlan).
So while the basis of the concern is valid, the current set-up is so far from making that possible that it's not really pertinent.