Good fortune!

With a doubled plate like Ken mentioned, the movement is limited by the fact that there are stones locking both plates in place.

Other than that, a cathedral's roof is tremendously heavy. Consider the complex framing many of them have using large sized timbers, and then consider the fact that many are then covered with slate, tile, or metal of some sort (tin, lead, and copper were all used at various points in time in various regions)

SO I would wager that the weight of the system is certainly a major factor.