I believe that's part of the strategy with Apple Arcade: every game will appear on ALL of Apple's platforms. You'll have your choice of where you play it, which in turn would stimulate greater use of the Mac and Apple TV for gaming.
We will see about Arcade... How cash will flow to developers and so on. I get the impression it will be more quality games there where simple ones will still stay on regular app store.
And as a random developer regular app store might be better. Only need to support one the one simply biggest segment (iPhone), can sell my app directly and get at least 70 cents per user and can still capitalize more with in app payments.