Steam's cut decreases to 20% after a certain amount of money. Also Steam does a lot more to earn their cut than any other platform, by far -- for example, they do a lot of promotion for you, both algorithmic and through things like Daily Deals, for free, whereas on iOS it is very difficult for ad spend to not be a significant part of your budget. The rule of thumb I've heard is, for every organic sale you make, Steam's algorithm will get you one more sale. So their cut feels quite worth it.
A closer example is game consoles, whose associated stores also take 30%, and nobody seems to complain about.
A closer example is game consoles, whose associated stores also take 30%, and nobody seems to complain about.