Apple is under no legal obligation to license their material at FRAND rates, nor are they under any legal obligation to make you happy. So let's table that.
What does the App Store fee cover?
• Credit card transactions (~3%)
• Gift card transactions (10–30%)
• Absorbing the costs of credit card fraud
• Running the App Store, bandwidth etc
• Performing app review[1]
• A license fee for use of Apple libraries and APIs
• Having your App receiving the residual goodwill of being available in a store where customers know that Apple is constantly breathing down the neck of developers to do the right thing[2] and pushing the envelope of policies such as not allowing third party tracking by Facebook.
• Having your App for sale in a store where customers feel comfortable generally need to fear credit card fraud, malicious code, where scams are less common (and can generally be refunded), etc etc.
Let's imagine that Apple said "okay, you can have an open market" and replaces their App Store fees with a license fee of 15 percent of gross revenues, similar to how Epic charges developers for use of Unreal Engine. Now you can have an open market for apps. Let's see how alternatives compete for the costs of running and marketing their stores.
[1] Just because something is imperfect, doesn't mean it's useless.
[2] This alone is worth 30% to me personally.