If the logic is standalone (e.g. algorithms, data structures, validations etc.), then that's indeed a really good approach! But in my experience, you oftentime

If the logic is standalone (e.g. algorithms, data structures, validations etc.), then that's indeed a really good approach! But in my experience, you oftentimes have to work with libraries and ecosystem components which are platform dependent. In those cases, the cross platform code would only work when these ecosystem components are also written in that particular language. Now, it can work with Xamarin Forms, React Native or similar technologies with very specific use cases, but it feels like any other, more generic attempts at getting something like that working are doomed to fail (e.g. when you don't have a cross platform solution in place, for example, different ways to access DB on different platforms, or different ways to make web requests etc., or even interact with device capabilities or handle permissions).