Free agents are like hired guns.
Think about it. They do well with a team for a number of years, hit the market, and then (usually) sign the best contract available. These offers come from teams whose needs can be filled by them.
In the case of the New York Yankees, CC Sabathia was just that. He came to New York on a long-term deal ahead of the 2009 season. It was just assumed he would show up, post great numbers, and hopefully win a World Series or two along the way.
Except, something different happened.