You could invent the best possible app store and take nothing as a cut and get no users because Apple wouldn't permit your app store and so you'd lose.
Apple uses their hardware market dominance to control the software market and extracts rent from people who want to sell software on their hardware. That's what people are complaining about.
"Why don't they just invent their own hardware, phone OS, and app store?" Hmm, yes, why not?