A common explanation is that there's easy ways to ward against guns. But now we're just doing Rowling's job for her. Honestly, probably better than letting her do it.
Considering the original meaning of “hit points” referring to how many shells a given ship could take before sinking, 12hp out of a basic spell is still pretty impressive.
In chainmail, the war game dnd is historically based on, a hit point is a fraction of a hit dice, each HD represents one soldier, roughly, so 12hp being the average of 4hd isn't too bad, no.
