It's the way it's been basically forever. Consoles take a similar cut or higher, or with high associated costs such as manufacturing costs, costs for devkits and tools. Retail markup in stores is similar, but perhaps more variable. Spare parts for your car or washing machine might only be available from the product manufacturer at a healthy markup over costs. There's nothing illegal about it, even if Apple were a monopoly. And it's not a monopoly by any rational measure.
Epic's argument seems to be that they should be able to put their app store on iOS, and let other developers use it and pay Apple nothing. Apple are not going to agree to that. But the fact Epic are also suing Google shows how crazy their thinking is. There are lots of app stores on Android already. Epic could launch one tomorrow, if they wanted. Unity just did.
Why don't they? As they tried it on PC and it's a massive failure. They thought that a competitor to Steam, but with fees cut by 60%, would be a runaway success. But it's fallen flat, with only a fraction the number of games of Steam. It has done nothing to dent Steam's popularity.
As a developer I would of course like to get more than 70% of revenues from app sales. But I can't see it happening through the courts as nothing Apple is doing is illegal. Maybe if Joe Biden is elected and appoints Elizabeth Warren head of a new Digital Markets regulator, the government might see fit to shake things up. Otherwise Epic seems set for an embarrassing and potentially very expensive loss.