Epic can't afford to sue every store, even if it wanted to. Nor would this be legally helpful for anyone - it would just enrich the lawyers. It makes sense to have a main case to test the legal doctrine.

Apple is by far the most distinct case, so it perfectly reasonable to focus on Apple (Besides, I'm sure that Apple with its trillions can afford a legal defence). If Apple gets off, all other stores get off too since Apple is the most restrictive. If Epic wins, we can then discuss how to apply the legal doctrine to other cases.