That's because it takes a huge amount of power to display the second hand. It may not seem like it, but it means the watch's internal circuits have to "wake up" 60 times more often for each passing minute. Think about that for a moment...
Thanks N4Newbie. That makes sense, but it would be nice if they had an option to set a timer for a certain number of seconds before it goes into ambient mode.
When I had my Zenwatch I remember getting an app that would control how long it would take before it would go to ambient mode. Might be Staylit Wear. Not sure if that app works on this version of Android Wear but you might want to check it out. Battery will drain quicker.