Copying isn't theft (though it may still be illegal). Theft is "dishonest appropriation of property belonging to another with the intention to permanently deprive the other of it."

So if I steal your wallet, your phone or your car that's theft because you no longer have access to it and I do.

If I copy a book, a CD, a DVD, a document or a chip the original owner still has it. That doesn't mean there's no prejudice or that it's legal but it's not theft.