Yep, I'd hypothetically economically gladly build an alternative app store charging "only" 5% on app revenue. The blocker would be that Google has a hold on what store is installed by default on Android. Apple even more so. This is hardly a fair market.
That would work because someone else "Google" is doing all the development of Android etc etc. Of course you can create a parasitic service for less.
There are other app stores on Android, e.g. OEM ones like the Galaxy store. The latter doesn't really compete on price though - 20% cut of one time payments and 15% of subscriptions. I think the Android app stores anchor to the iOS app store prices to piggy back on Apple's monopoly power - devs are used to having to pay Apple that much, so they will not resist if you charge the same.