I'd guess it's twofold:
1.) Lack of fiber in the ultra processed foods
2.) shelf-stability additives so the food can sit on shelves for months and not go bad.
But the authors of these studies are trying to ban UPFs in several contexts and enact further policies that highlight them as specifically dangerous. A bunch of fine foods being swept up in this is a concern.
I remember seeing something about how carrageenan, a common ice cream thickener and emulsifier can cause cancer and is banned in the EU but not the USA. I was bummed when I checked my favorite brand and it actually had it as an ingredient.
