This comparison doesn't work at all. An APK for app A is compatible with Android devices of version X, regardless of the store it is sold on. A cosmetic for game B is not compatible with all games running on the same engine Y, for obvious reasons.

Asking Fortnite to accept other stores selling Fortnite-compatible cosmetics doesn't work either because Fortnite has not monopolized a trillion-dollar industry, meanwhile spending billions on lobbying to make daily life for the average citizen impossible without them, which the Google-Apple cartel has. Fortnite has also never gained market share by pursuing claims about being an open source platform or not being evil, again unlike Google. These differences.. make all the difference. Call me when my kids are forced to agree to Fortnite EULAs to participate in schooling all around the world.