It's actually not a duopoly, but two monopolies:
If you're on iOS, you have to eat the Apple App Store cut.
If you're on Android, you have to eat the Google Play Store cut.
They don't actually compete with each other in any meaningful way, so there's not even price competition between them. As long as they're able to control their respective platforms and ensure they're the only successful app stores on them, they can charge whatever they want, and have no incentive to lower their rates.
