The NFL doesn't work like that. You can't extend every player that plays well.
There is a good chance Warren would not agree to an extension because like you he realizes his value could go up in the coming months, so why would he sign an extension while his value is still down? Players aren't stupid.
Extensions can be detrimental to both the team and the player. Maybe the player's play tails off after the extension for whatever reason, which has happened this year with Ike Taylor, or a player ends up getting a contract that is way too low because they didn't wait it out and let their value rise.