Well, I'm gonna try to make this more than a GameFreak made it that way Answer.
Notice how Arceus is considered the Pokemon that Created the Sinnoh Region. Well, if that were the case, then he, or some other Pokemon that he created, must have created the other regions. Or, it could be due to the fact that since he is the creator-pokemon, he should have something made in his honor.
Also, here is a Map or the region:
Notice how all the regions look very similar when compared to their original games. This could have been a plan that GameFreak has had for a long time.
Or simply, this could be the specific map the GameFreak has made for the world of Pokemon.