Back when Apple announced this there was a good discussion about it on Accidental Tech Podcast, about how this was a really smart move.
It gives a price cut to the most number of people, but requires Apple to give the least amount of money away. This gives them really good press, and helps them to deal with their "image issues" that they have at the moment. Basically a win win.
The stranger thing is for small apps Apple/Google is providing services of more than 30% I think. Basically they are managing everything and a lot of time also promoting it for a small price. But the apps that are already famous like fortnite, a user who install it will likely install if Apple/Google is not involved.
It gives a price cut to the most number of people, but requires Apple to give the least amount of money away. This gives them really good press, and helps them to deal with their "image issues" that they have at the moment. Basically a win win.