She can't kill them off because the demon queen's power grew off the charts, with her continually getting off with her husband, the previous hero who was "convinced" that the goddess didn't have humanity's best interests in mind.
Right now, the goddess and the demon queen are battling each other. Every new monstergirl created, and every region that is corrupted into a demon realm add to her powers. Before long, the goddess will be defeated, unless humans get their shit together.
Also, the other gods were corrupted or defeated by the demon queen somehow.