It's due to the way copyright law works in many countries. Fair use gives you the right to make copies of the media you own both as a form of backup and to share with your immediate family members (e.g. copying a CD for your spouse, parent, or child). Downloading copyrighted material doesn't fall under fair use, because you're not downloading a copy of the disc/cart that you bought yourself; you're downloading a copy of some other disc/cart that somebody else made and did not have the right to share with you, because you're not family. As such, downloading copyrighted material is always piracy. Whether you own a physical copy of the thing you downloaded is irrelevant.
Borrowing a friend's hardware to rip your own disc would theoretically fall under fair use, however.
Borrowing a friend's hardware to rip your own disc would theoretically fall under fair use, however.