I don’t understand. Android already allows other apps and app stores to be installed, and Epic already has an Android app store you can download and install without issue. What was the argument here?

Edit: tldr: apparently it is not good enough for Epic to have their own app store, they want to have their app in Google’s app store and still not pay them money for purchases made in the app.

Google paid off other OEMs to make Google Play the default app store (much like they paid off other companies to be the default search engine) which the court decided was anticompetitive.