Why is there no humanity left in DS3?

In DS3 you cannot consume humanity. You cannot become human. You are only Ash that can be embered. Why is there no humanity left? I have read that the flame has been dying out for so long that all humanity has dwindled and gone by the time DS3 takes place. what are your takes?

I habe not played the DLCs yet if that makes a difference so please dont spoil them for me.