"My guess is they manipulate the expenses so the 30% disappears"
Since both Google and Apple are profitable overall, there isn't any such manipulation possible. If they lump expenses into the app store that maybe don't belong there, those expenses can't be used again against some other income.
This kind of arbitrage is only possible / only worthwhile if you have different tax rates. Since a dollar of profit from the app store is taxed at the same percentage as a dollar of profit from selling an iPhone or slinging an ad, there's no point to doing this. At least I'm not aware of any countries that might tax the two differently, but I suppose some may do so.