I agree with your sentiment entirely. It’s worth noting though that at the time Northern Ireland (NI) was conceived, large swathes of the land that would be NI consisted of a republican (and typically catholic) majority. They still do to this day. The regions that did have a unionist (and typically Protestant) majority were really only the north east coastal counties - and even there, the unionist population didn’t have a significant lead on the republican population.
If it had been separated due to the inclinations of their respective populations, Northern Ireland would only have been in and about a third of the size it is today.