That’s not a good argument because what if you stole something from a store ALL the way in the back covered in dust that literally no one even knew about. Would you say that’s not theft because it’s not a loss of value?
Software and digital content piracy does not deprive any other owner of anything else, this is isn't stealing in the legal sense. It may be financial harm or copyright abuse but those are both separate crimes from theft.
Piracy isn't legal, but it isn't theft either.