Because they tend to cull themselves pretty thoroughly in their own quiet little tribal spats, especially when overcrowding becomes a factor. All the other races really have to do is keep them somewhat hemmed in and otherwise stay clear.

__________________
And you, to whom adversity has dealt the final blow
With smiling [faces] lyin' to ye' everywhere ye' go
Turn to, and put out all your strength of arm and heart and brain
And like the Mary Ellen Carter, rise again.