If you read Scandinavian texts that are a few hundred years old then you will discover that every man and his dog was engaged in the ancient sport of defining exactly which tree, steam, rock, etc marks the extent of his land and he will also keep a running record that goes back even more hundreds of years all the disputes etc over each square inch. Not surprising with all that going on there was not much time left over for mere county borders
But given the nature of the Cyrodiilian Empire it is very surprising that they neglected such a thing. The architecture seems very anglicised and Old English maps kept careful records of County Borders they can be found in antique shops here. There was the original Doomsday Book that Wlliam the Conqueror created listing everything in his kingdom starting shortly after his arrival in 1066 - and ofc the frequent baronial disputes - so maybe it is something that is connected to the climate and nature of the land as well as the structure of the culture?
Looking at Morrowind, Vvardenfell has already divided up quite clearly by the volcano - and to the Great Houses the Ashlanders don't count. I can't remember maps in Daggerfall that gave such info - likely because it was because the map was 24 times bigger and a lot of it was generically, randomly generated - maybe that has sort of survived?