Old-fashioned high seas pirating may have been stealing, but the modern copyright infringement form has never been stealing.
A key aspect of stealing is that you're depriving the owner of some kind of property. While you have that property, they don't, and they can't use it. Copyright infringement doesn't deprive the owner of anything. The only thing they lose is the government-granted monopoly over the right to distribute that "idea". If copyright infringement is like an old fashioned crime, it's like trespassing. The government granted someone the right to control who has access to some land, and a trespasser violates that law.