Because La Russa clearly isn't going to use Westbrook when the Cardinals are ahead, no matter by how much.
Let's think about this ... Heading into the bottom of the seventh inning, you're ahead by eight runs. You've got two more games in the next 48 hours, and your No. 4 starter is pitching the first of them. Both games will be played in one of the game's friendliest places for hitters. There is every indication that you'll need your best relief pitchers to give you some good innings, in these next 48 hours.
Given all that, would you let Lance Lynn throw (another) seven pitches in the bottom of the seventh? Would you replace Lynn with Octavio Dotel?
Tony La Russa obviously would, because he just did. You would, maybe. I would not.
I say there's little point in carrying Jake Westbrook if you're not going to use him in exactly this spot. He might not be good. But there's 99-percent chance he's good enough to not blow an eight-run lead.