The builders were not unaware of corking. The chamber in Newgrange is waterproofed with sticky stuff between the stones.

I personally favour the partial coverage up to the capstone theory for most of them. The thing with this is that there is no definitive evidence to support any theory. If the lack of absolute evidence is evidence then the evidence says the tombs were not completely covered because none exist that are completely covered.

Oh .. and the majority weren't tombs ... bugger, there goes the apple cart!