I mean... The apps are already single platform only. iOS apps are written in C#, Android apps are written in Java. They are not in any way compatible. If you want an app to work on both platforms you literally need to build it twice. It's not twice as much work, but it's nearly that. And if you only know how to develop in one of those platforms, it's a lot more work to learn the other.
I think demanding something work on multiple platforms isn't really a fair requirement, especially for smaller developers, and it would likely result in fewer apps existing at all.