Right, but the value isn't that you're listed in the app store, the value is that you can collect payment on-device. The store isn't the retail space, the whole device it.
You could not release an app in their store and ignore their existence entirely. But that's ridiculous and you can't afford to do that because their user-base is so valuable. And so Apple charges a premium because they know that.
If Apple had unlisted apps that didn't show up in the store listings and required a click through from your website to get to the app's store page, I bet some developers would opt for that if it meant they didn't have to pay the 30% tax.