This really isn't the case. I have built apps with lots of code reuse on native and web.

Abstract all data logic and you can use the exact same code for both apps. This of course does not include your view layer. If you do it correctly, all you have to do is make a new set of view components. Stateless ones. Because you are injecting the state from your non platform specific components.

It makes it extremely fast to build for other platforms. It just takes a few apps to fully understand how to abstract these things perfectly.