When the realms collapsed onto the human realm the land changed to accommodate them.

 

As mana flooded the land and realms merged with each other and the physical landscape of the human world, chaos ensued. The creatures that now inhabited the same earth as humans destroyed many of the civilizations.

 

There are still several major cities that were able to hold out, but those left are in disrepair and full of Bolas' corrupted henchmen enforcing his 'new order'.