Gods oversee the world.
Much of the world is untamed. Also, much of it is unmapped.
The world is ancient.
Conflict shapes the world's history.
The world is magical.
There are no atheists in the Forgotten Realms.
Gods exist. They give power to their clerics, who uphold their teachings. It is still in mortal memory when the gods actually walked the land.
Invasions and disasters dramatically shape the land. Each event causes civilisation to draw back. When the dust has cleared, people move out of the safe lands and resettle the new wilderness. This cycle has been going on for several thousand years.
When civilisation pulls back, knowledge is lost. Often the resettlement also involves relearning what is there. Sometimes, of course, the disaster changed what is there. The Time of Troubles, for example, made some major changes to the geography of the land.