Maybe all humans should speak 1 language. Maybe there should be no countries. Maybe we all should be the same color and have the same culture. While this may be an interesting idea to discuss over a beer or two, it is so far removed from how the world really works that it's just plain stupid to use it as a argument. Wishful thinking is great and all, but this is never going to happen, and especially with the multiple decades of legacy, disagreements over usability and user experience, non-compatible accessibility systems and API's, the different platforms are very very far from each other.

Now I understand that accessibility is most likely not a high concern for a React dev, but there are people out there who want to give the users of their platform of choice the best possible experience, and that best experience would be delivered with said platforms native tools, because they have the best integration with that platform.