Let's compare this to something else. Take Amazon. You are a 3rd party seller wanting to sell your goods. You really think you have the right to say hey Amazon Im going to sell my stuff on your site, that you pay to maintain and used your money to build its reputation to what it is now, and not give you 1 penny? I mean good luck I guess?
You want to play in someone's sandbox they built up on their money and consumer capital then it is pay to play. That is how the world works; nothing is free.
Now you can argue 30% is too high or not, but all devs should have to pay Apple SOME piece for using their store and customer base Apple built up on its own money.
Unfortunately, your comparison doesn't hold any grounds whatsoever. Because unlike Amazon, Apple leaves you no choice. As a dev, you either put your app on Apple's store or nowhere else. To stay with your comparison, a manufacturer of goods has a choice to sell their goods wherever they like. Amazon, other stores, their own stores, literally anywhere.
None of this has anything to do with Apple charging too much money. It's all about choice. Which Apple does not provide, and that is clearly an antitrust issue.