The Giants had every intention of adding a frontline starter this winter, and they made Jon Lester their target. Lester chose Chicago instead. Max Sherzer probably offers more upside, but he's seeking a contract in the upper stratosphere, so James Shields was the best of the next tier. And the Giants apparently don't want him, despite the fact he'll likely sign for $50-$70 million less than Sherzer will.
And you know something, maybe we should all probably be pretty ok with that.