As a developer, I made a couple of hundred thousand dollars from the Apple App Store. You know how much value the App Store provided me? Way, way more than the 30% I paid (this was years ago).
Without a portal that could direct millions of people to find my app, I probably would have made tens of dollars.
So we can debate fair pricing, but the idea that aggregators don’t provide any value isn’t an indictment of aggregators, it’s a confession of not understanding the business.