But there were no such things as "countries" in the Demon Continent.
Whenever there was an important matter that needed to be settled, representatives from each family would meet together and decide on a course of action.
Even if they wanted to consolidate into a country, the Demon Races themselves were made up of a wider range of clans compared to other races, so it was difficult.
Mermen and birdmen can't possibly be expected to adopt the same way of living. It was also impossible to force them into the same way of thinking and doing things.
For those reasons they could not come together as a country.
In the eyes of the other races, they were uncivilized barbarians.
There are even those who look and act similar to monsters.
Then perhaps it was inevitable that discrimination progressed into war.