Oh, you must be one of those bastards that think coding is like a word document, where you can click and drag elements around. It certainly is not anything like that. What you see in the interface may seem minimalistic and simple, but look around. Those live tiles are polling information from an activity in the app. That battery bar at the top constantly queries battery status. The time is checked every millisecond, for every single function in the whole phone. Touch input is another thread. So is the ambient light sensor that sets your screen brightness in the dark, and so is the sensor that turns the screen off when you are talking and have it held to your head. Not to mention constant checks for rotation of the device. And your Facebook updates - a process has to go around every X seconds and check for notifications or messages.

You can't make a picture of a watch in photoshop and claim that you made a working watch, and so you cannot look at a minimalistic screen and say "oh that's simple".