They've spent their lives in obedience to canon law.  They believe the leaders of the Church, the hierarchs before them, speak for God, hold the keys to the kingdom -- ultimately the Holy Spirit will not let that leadership be wrong, as a group.  And if some few of its leaders have been wrong, they would be betraying the Church -- God Himself -- by admitting it, as they see it. 

We certainly hear defense of the magisterium and its dictates from some of our more conservative commenters here -- imagine how much stronger those convictions must be in the hierarchs with a vested interest in not being wrong, a conviction that they cannot be wrong, or at least cannot admit wrong.

(Excluding, of course, the ones who have lost all faith and don't really believe in God or an afterlife.)