You cannot steal what is not physical. Theft implies removing a physical object from somewhere, creating a loss of an item.
Digital information is 1s and 0s, and you just create a copy. You do not remove the original one. There is no theft taking place.
The value of a product does not go down because I didn't pay for it. If anything, if it's a quality product, the value goes up. If I pirate something and enjoy it, chances are I will pay for it when I can afford to. If I pirate something and don't enjoy it, well, I wasn't going to buy the product anyway so there's no loss. Let's say I watch a movie at a friend's house and absolutely hate it. I do not buy the movie. How is that different than pirating it and coming to the same conclusion? I see the movie without paying money.