Primarily because Kotlin is not a superset of javascript and thus does not inherit the quirks of javascript.
Other than that, if your backend is kotlin then you can potentially share (non JVM dependent) business logic code with the backed. Same goes for android clients. Same interfaces can be be used in client and server.
I've never really found a good use case for reusing business logic, but just reusing types is pretty helpful. Although you can also get that via something like Swagger as well.
I too am more of a fan of the Swagger/OpenAPI approach (in general terms), but I find it valuable to be able to share validation logic too. (Unless your validation requirements are extremely simple.)
Of course Swagger/OpenAPI doesn't preclude using the same language on the client and server side, so one could always combine "isomorphic" approach with the "service definition" approach.
Other than that, if your backend is kotlin then you can potentially share (non JVM dependent) business logic code with the backed. Same goes for android clients. Same interfaces can be be used in client and server.