Because it's not stealing. Stealing is a problem because it deprives the original owner of the item - whether the thief subsequently uses the item or not doesn't change that.

This doesn't apply to dematerialized content: the original copy still exists. The only negative impact occurs if someone decides to actually use the pirated copy in place of buying a licensed one.

The mere existence of this new pirate copy being around doesn't automatically imply that, especially if other, more convenient sources are available.

Okay, call it copyright infringement then if you want to be a stickler on definitions. It's still wrong and existing instances of it doesn't make it justifiable to do.