But the monsters they’re fighting in this book (redcloak and Xykon) have huge amounts of backstory!
And in the last book, they fought Crystal (with a backstory), Durkula (with a backstory), giants (with a backstory), and.. the poor wyrm (who admittedly didn’t have much of a backstory, but still managed to have a tragic character arc.)
The only character who showed up without any reason or explanation was Not Thad, who is not a monster.
I’d say Not Thad disproves your hypothesis that people have backstories, but monsters don’t :-)