Stack Exchange is about learning. People come and ask about their problems. Others help by answering them. My Question is why Stack Exchange is not planning on adding some fun like games, quizzes and other such competition, where one can test their knowledge?
The purpose of Stack Overflow is to build a knowledge base, to be a one-stop site for real-life programming problems.
It is not meant to be a place to hone your skills, although being active on Stack Overflow will definitely teach you new, useful things.
If you want to exercise your actual coding skills, or even compete with others, there are plenty of other sites outside the Stack Exchange network. Like Topcoder or Project Euler.
Edit: As @Stijn pointed out in the comments, within the Stack Exchange network there is CodeGolf.