That's interesting - I wonder where you are? Our ca. 1800 cottage has no foundation (it's literally just rocks sitting on dirt) and is in a pretty wet climate (Ireland). Though the room that had a screed put down is drier than the others.
East of Lake Huron in Canada. I think the difference is that the average winter temperature in Ireland is above freezing (correct me if I'm wrong). It's about 20C lower here. It's the freeze/thaw action that we are building deep to protect against.
I'm not an architect, but my understanding is that the foundation has to be far enough down that the ground does not freeze under it, which is why houses in the Northeast US and Canada have basements.
Go farther South and it gets down to eventually nothing. Go far enough North, to permafrost country, and you not only need to dig down to the permafrost but you need to have insulation so the permafrost doesn't melt.
It's actually calculated based on the frost line for the area. I don't think when building on solid rock it matters, since the point is to avoid expansion and contraction of freeze thaw cycle pushing the house up.
