the unauthorized use of another’s production, invention, or conception especially in infringement of a copyright

Piracy can often invovle legally obtained items or even bypassing item bans.

You pirate a movie by taking a real copy and sharing it with others over the internet by making more copies or by making it copyable.

Stealing a movie would be taking the real copy without paying for it.

You could both steal and pirate the movie, but in the context of modern media, the source material is usually obtained legally.

That’s why most torrents usually have the source in the title to show what it was taken from (DVD, BluRay, WEBRiP, WEB-DL, etc).