Because it doesn't help.Tom Cashel said:
For the hundredth time, HEAL! Why doesn't Heal count?
Harm is effectively a save-or-lose spell; if you get hit with it, you're gone. Before you have a chance to cast Heal, you will take at least 1d4 damage from the cleric or one of his allies, and you will be dead. Once you're dead (or dying), you can't cast spells.
An opposite spell is not necessarily a counter.Harm's specific check, balance, whatever you want to call it, is Heal!
Think about this one: what's the balancing factor for Slay Living? It sure as hell isn't Raise Dead, which is the opposite spell. You can't even raise someone who has been hit with SL. The correct answer is Death Ward; it protects against SL before it hits.