Both the Appstore and Google Play do literally nothing for you, except distribution.
I mean distribution is not “nothing”. They have to maintain the app store, and process payments, and filter (most) malicious software.
It’s just not worth anywhere near 30%, not to mention the flurry of ads all over the place.
At least with Android it’s completely possible to have a third-party app or app store and charge using their own payment processes without ever touching the Google Play store.
Apple is a whole other level of control and anti-competitiveness, and they’ve been profiting off of it for decades with no intervention in sight.