Apple builds trust in the iPhone by curating apps that are allowed to be installed on it. When I buy an iPhone for my family I _trust_ that Apple is doing the right thing and will prevent its users from harm. I wouldn't have that same level of trust if it was easy to just add alternate App Stores on the device.
Apple curates and still bad actor apps make it onto the app store. It's not like Apple has a 100% pure record here. There is also no reason to think that Apple is the only company in the world capable of carrying out curation. If Epic Games, had their own app store I doubt they would want to allow apps on their app store that violated their users' privacy and other malicious activities just as much as Apple. Maybe you personally would never feel comfortable trusting Epic Games, but why should others even be denied the option?
Apple owns the majority of the US mobile market, so as a US consumer I could choose to go with Android, but then I won't be a part of the blue bubble crowd with all my friends. Also, Google has much the same policies with the Google Play store, so there really isn't a choice. It's a duopoly of bad implicit choices. Apple is able to charge 30%, because they can exploit their market position.
