Due to the same reason we cannot go over the edge today. There is no edge. There is a 1000 foot, straight up and, who knows how deep, ice wall all around the perimeter of a plate shaped flat earth... according to their belief.
This wall holds all the oceans in and whatever is beyond it is only known by the people on "operation High Jump".
IDK, maybe Sheol?
IDK, it wouldn't need to be that deep since we cannot drill very deep into the crust until it gets so hot our drill bits melt or get so hot that they lose their integrity and cannot drill anymore.
Some of the deepest mines are so hot that you cannot stay down there without conditioning yourself and then with proper hydration.
Basically, when it comes to the earth surface, we cannot scratch the surface.
Same reason ships don't, I guess.
I don't believe it does, in their view.