Thoughts on Cathedrals

I've been doing some reading recently about the Gutenberg myth; is the printing press responsible for the modern world? Some other theories exist. One of the most interesting is that the stirrup is actually a more imporant innovation i.e., the stirrup created mounted horseman; which created the knight; which created a priveleged social class and a two-tier society; which empowered the knight's life of training and tournament; which led to heavier arms; which led to improved mining and metallurgy; which led to more armour totally covering the knight so that personal insignias were required for identification; meanwhile the improved metallurgy led to improved agricultural technology such as the heavy plow; since a heavy plow required a team of eight oxen; communities began to organize as collectives of individuals representing an alternative authority structure. NOTE- these aren't my ideas but I can't seem to find the reference I'm looking for.

After all this reading, I woke up this morning with an odd thought: what exactly is the purpose of a cathedral's flying butress? I know the normal answer is that the butresses are required to take the lateral load of the roof so that the cathedral walls can be pierced with windows. This explanation implies that a torque force (or moment) is being applied to the walls. Why not just support the roof simply on pivots so that the entire load is passed in shear? Why didn't those famously brilliant cathedral builders hit on the idea of a simply supported roof?