Why else part is working well but not if Part, even when I am on mainThread and I do not need to use Handler to post the task to MainLooper

Question

Private void execute(final Runnable runnable) {

if (Looper.getMainLooper() == Looper.myLooper()) {
    runnable.run();
    // Here I want to update on UI but it does not update or run UI. but when I use handler like in else condition, i see cahnges in UI
   // I want to as when we are already on UI thread , why we can not simply see the result here but in else part
} else {
final Handler handler = new Handler(Looper.getMainLooper());
    handler.post(runnable);

}