"...absolutely nothing to do with conceiving, designing or developing such a successful game."

But it has everything to do with distributing the game. Also it has a lot to do with outsourcing the maintenance of the mobile platforms — designing hardware, OS releases, etc. Epic did not design their own hardware, their own silicon, build entire global supply chains, design UX, etc. Apple and Google did. Do they not deserve a cut?

I'm not here arguing the percentage of the revenue, just that that some percentage is clearly warranted here. Their relationship with Apple and Google is clearly a partnership, meaning it has to benefit both sides. Same way it works with consoles.