While I'm not really sure, I think that after the last Xamarin.iOS update, my XS studio won't let me select which simulator I want to run.
I have made a few research and tried everything I found, so far no success at all.
I have tested with my Xamarin.Forms project (with the iOS app as startup project), I have made a new XF project and even tried a simple iOS project.
I have completely remove Xcode and Xamarin, reinstalled it all and the problem persists.
Ah, I have the simulators installed and running perfectly with Xcode.