Board Thread:General Discussion/@comment-25266931-20161228230634/@comment-27082610-20170102150841

Because the two brothers never removed anything from the world, only created something to fight back against what the other had created during their shift. It's possible they simply didn't remove anything to make things more interesting.

And in regards to them only targeting sapients now, well Salem is clearly controlling the horde now, giving them a more specific goal. And the series lore is still in its adolescence despite all the exposition we've gotten this volume between the last episode and WoRs that were actually helpful. They haven't really had time to explain everything yet.