On the other hand, spells in general are balanced around the idea that you can only expend them so quickly. That's the action economy at work. Spells that are reactions are both more powerful because you can cast them essentially without spending an action, and more limited because you have to wait for the trigger to happen. However, it also means that you have to make a hard choice. Do you cast Counterspell, Shield, or Absorb Elements? You can't do more than one. Breaking that restriction drastically increases the potency of reaction spells because they suddenly actually have no action cost and they have triggers like "you're hit by an attack" or "someone casts a spell". Those triggers happen all the time. None of these spells were designed around that.
There are a total of six spells between the PHB and XGtE that are reactions, and in my experience the three I mentioned are a routine component of every spellcaster's arsenal who has them on their list.