Code reuse - I have an dual iOS/Android React Native app and over 90% of the codebase are generic components. Most of the difference are simply stylesheets, for platform-specific UX.

Besides that, there's the incalculable benefit of being able to use the same mental model (to say nothing of programming language) when developing, especially when designing UI components and animations. Even if you had equal skill with building layouts in both systems, there's great advantage in not having to context switch.