Because there would be no point in ending it if he's healthy enough to play.
Let's not go all Espn here, some of you, the game in Dallas (which those of us who grew up seeing Favre get stomped in that town knew we should dread) does not somehow disqualify how he played all the weeks previously.
"Wow, Favre played poorly & got hurt in a game where we should have seen it coming...bench him!"