In what way are they not designed as such? The biggest issue I can think of regarding major refactoring is coordinating across the team and ensuring you're collectively at a good point to start changing things. That's not really a language/tooling issue.
For everyday operations and refactoring, I find the tools to be the limiting factor.
I've just finished a contract where we had to deal with legacy code on iOS and Android (yes, it exists. Let two junior programmers not very keen to learn from the outside during 5 years working on the app and see with what you end up).
Refactoring the Android code to a healthy state was greatly facilitated by Android Studio and the whole IntelliJ suite. We were not afraid to move things around and could always see the light at the end of the tunnel.
On the other side, XCode has been a pain in the ass for the whole project. We had ObjC and Swift coode. Good luck with that. Even the pure Swift was hard to refactor. SourceKit is great and should help to get a top notch IDE that can resonate about source code but somehow Apple does not seem to care about the tools.
Sorry, I get passionate when talking about refactoring and tools.
Another example is how the Pharo Project (http://pharo.org/) is refactoring the Squeak source (http://squeak.org/) code/architecture. They developed custom tools for this purpose and their code cleaning is quite impressive when you know where they are coming from.
