Scaling all UI elements when using an external monitor
The OS X UI is not resolution-independent, the only thing you can do is to change the resolution. Which is essentially what OS X does on HiDPI screens when switching to Larger Text or More Space, but since they are retina displays, you can't see that the resolution in some cases is not native, which you definitively can on non-retina displays. 

There is a terminal command to change the interface size (which I can't seem to find right now, but I have tried once in 10.6 I believe), but it doesn't work well, especially in 3rd party applications. Some are absolutely unusable.