So, do you have a ruling somewhere that states that copies of copyrighted works can be regarded as "stolen property" for some other purpose?

No because those other related purposes are generally applicable to larceny specifically, as opposed to other crimes of theft.

Why are there completely separate laws regarding theft of physical property and the violation of copyrights if they can be regarded as the same?

The same reason there are completely separate killing laws, or drug laws, or property laws, or environmental laws. We’re not limited to one single law that covers an entire category of crime. State and federal governments pass new laws in existing categories every day. Thats why being a lawyer is hard, there are a lot of laws and the way they interact is complicated and inherently modular. Just like there are different kinds of those other crimes, there are different kinds of theft, so you need different laws for each kind. Larceny or larceny-theft, embezzlement, fraud, identity theft, copyright infringement. All theft, different definitions, requirements, circumstances, punishment, interactions with different laws (Receiving stolen properly, transporting stolen property across state lines etc.). You’re just latching on to larceny-theft, one very specific kind of theft, and mistakenly assuming that it is the only theft related law we have on the books.