So I'm going to preference this with a huge IMHO.
It's just easier that way. The average D&D party is cosmopolitan. It's probably a good thing to say that represents society in large rather than being the exception. It's not like conflicts can't still happen; nationalism doesn't need to see race in order to function. Furyondy doesn't need to be overwhelmingly human to war with the Empire of Iuz.
Moreover, most species historically didn't have their own monospecies nations. There is no halfling nation or gnome nation and nobody questioned where they lived or how integrated they were in other lands. If they can just live amongst human nations, I don't see why dragonborn or goliaths can't.
Lastly, and on a personal note, the real world is so full of division based on race, gender, religion and lifestyle that it's mildly refreshing to imagine a world where that doesn't matter as much. I'm kinda over characters being hated for being born an orc, tiefling or drow by society writ large. Let hatred be a personal, rather than societal ill for a change.