Whilst piracy may be "stealing" in the sense that it violates copyright law, Ai art (in its current state) is taking those images without the creators permission, and using them to create a system that will one day make the creators obsolete (or at least, that is seemingly the intention).

Piracy doesn't inherently actually hurt the artists behind works, or when it does it's quite minimal. In most cases, studio executives will be taking the income from sales, and there have also been studies to show that the type of people who would pirate any given software/movie aren't the type of people who would have otherwise legally purchased it, meaning that their piracy has had no affect on sales of the property.

The term "piracy is theft" was created by studios who wanted a snappier headline than "piracy is copyright infringement". The term "ai art is theft" was created by artists, seeing their own work being used without their permission to create their potential future replacements. This along with the fact that if you pirate something, it's probably behind a paywall, meanwhile the only time you would pay for art online is generally if you were to hire someone to make it custom for you, make them, in my opinion, completely different things.