These apps are streaming massive amounts of data from epic's servers constantly. The incremental cost of downloading the actual app code is the tiniest of considerations. The app is a couple static files that would be served from a CDN. Now compare this minuscule cost to giving away 30% of their in app purchases. There is simply no comparison.

Bandwidth costs next to nothing these days.

And it is also rarely if ever measured in petabytes. Commercially percentile based (in terms of speed) billing is the norm, but that only applies to businesses that act as downstream customers of ISPs

Apple has global IX presences and generally maintains open peering policies, which means it only costs a few bucks monthly to maintain any given PNI (e.g. 10Gbit), and they are also available on those open routing server ports. IX presence is dirt cheap.

As far as I am aware what gets downloaded from the app store is little more than the launcher, which then downloads the actual game files from epics server.

Why shouldn't Apple compete on pricing against others then? Drop the arbitrary %, charge for actual usage. If they're so good and cheap then everyone will stay with Apple distribution.