Companies by definition exist to make profit, while countries by definition do not exist for profit. It makes sense that CEO #A is willing to give up his role as a CEO for personal profit, while president/prime minister of a nation state will loose power getting nothing in return. Add to the mix all those nationalistic/patriotic/other reasons and you are in eternal status quo. I could actually add very good case study for countries not merging. Cyprus and North Cyprus, divided since 70ties, they tried to merge back together for 2 decades now. With huge economical gain for North and political for South, but Turkey does not approve the move hence, despite majority of population supporting idea it's getting nowhere. Another good case is union of Belarus and Russian Federation, despite being knitted together (economically and culturally at this stage), president of Belarus is nowhere near to accept to be another federated state of Russia (and basically being appointed by President of Russia). I would say ambition, grudges and corruption are 3 main factors while states are not willing to merge together. That being said, there are economists that claim that merges between corporations are most of the time pointless and only a device to bump short term stock valuation.
But it's funny, and while their sarcasm or satire might be not for everyone taste they don't exclusively attack right-wing people they do with everyone that says or does something not very smart one way or another -> https://rationalwiki.org/wiki/George_Monbiot