Digital BW, The Print

Last question of on this topic.

Maybe this is just a rhetorical question, but, since the aspect ratio of 33mm and dslr's is 2:
3, why don't pre-cut mats follow this ratio?

Is is because medium and large format cameras have different aspect ratios? Is it because 
the art community (painters) have traditionally used the sizes provided?

The guys at the local frame shop didn't have an answer.

Thanks again,

Steve