Because it’s one Country, a line was drawn by the British through the country dividing families, farms, houses and villages. Its an artificial line drawn to ensure a sectarian state existed.
Interestingly, that should have been all cleared up - but the Boundary Commission's report was leaked - and it caused outrage in almost all quarters. As I recall, a lot of the absurdities were due to be eliminated, but because of the Nationalist population going mad because they didn't get the huge swathes of NI that they wanted, and the Unionist population going mad because of the loss of territory - it just couldn't happen.
There were plans to return areas such as South Armagh to Dublin - but, something that kept coming up - would the Republic really want such places? Ireland couldn't afford the 2bn+ subsidy given every year to NI, for a start.
In the North the Unionist government built a motorway to nowhere, not to the second largest city in the north Derry or Dublin the largest city on the Island but to a field 15km from the nearest town. This is an example of bad governance that union with Britain brought.
That was a fantastic example of how completely out of control the Unionst Government was in NI. The state of the road between Derry and Belfast to this day is a disgrace - but they do seem to be finally doing something about it.