We've had to invent the term because "junk food" isn't well defined or understood. A lot of ultra processed food wouldn't be considered junk food by many people

We've had to invent the term because "junk food" isn't well defined or understood. A lot of ultra processed food wouldn't be considered junk food by many people and some junk food like cakes and pastries might be considered processed but still not be considered "ultra processed". As a new term, the definition of "ultra processed" is still being narrowed down and clarified, but at least it isn't saddled with several decades of past definitions or ideas of what junk food is or isn't. It's become clear that ultra processed foods have major negative impacts on health in ways other foods don't. My guess is that as research into ultra processed foods continues we'll eventually end up with even more new terms to drill down into specific types of foods, production methods, and even marketing practices that are harmful/unhealthy.