It's literally the legal definition. Copyright infringement has never been theft. Media companies have been trying to change the definition of theft, though.

It used to never be a crime, only a civil offense. This means the rightsholder has to sue you, rather than the state prosecute you, but also that the burden of proof is "the balance of probabilities", ie whichever side tips the scale past 50/50 with their argument, rather than "beyond reasonable doubt" which is more like >99%. However in the last decade many countries have introduced "commercial copyright infringement" as a criminal offense. Off the top of my head, in the US I think the threshold for that is like $1,000 or something.

It's not about it being "harmless" but the fact that you're not taking something away from someone. If I steal your laptop and sell it, you no longer have a laptop. If I copy data, you still have your original copy.

This is also why there's a different crime for "joyriding" instead of just stealing a car. If you steal a car, you might argue that you were just taking it for a drive, and never intended to permanently deprive the owner. In that case it's easier to convict you for joyriding instead of theft.