Consumables need gold to purchase whereas abilities are free to use.
The same argument you make can be applied to healing potions and Clerical healing, or Alchemists' Fire/Acid flasks and Wizard fire or acid spells, just for example.
D&D has always been a resource management game to a greater or lesser degree, and having to buy consumables to create effects that certain classes get for free doesn't seem unbalancing to me.
