Because on his day, he's invaluable. As rare as those days are. He's worth keeping for the goals.
But if you are running a football club you have to weigh up how valuable and over how many years. Would he be valuable enough to keep in the first team next season and not have bought Cabaye?
Nope, always thought we needed a CM upgrade. But how often have we played our 1st choice side this season? We need options and he's a pretty decent one to turn to when you're chasing a goal.
Which is probably why a contract offer had lots of appearance-based incentives. And probably why he turned it down.