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.