Just because they are accessible, doesn't mean using them is seamless. For example all of Objective-C's runtime and classes are available to Swift but there is some friction e.g. methods that emit untyped NSArrays rather than typesafe swift arrays, or cases where you have to avoid using Swift structs because they can't be passed into the Objective-C runtime. Apple's done a lot of hard work, and each release of Swift has made this better, but it's still not entirely without hiccups... without having used Kotlin, I can't say for sure, but I would guess there are some analogous issues.