@reu
Good reasoning, but it's already been established in-universe (by Frieren IIRC) that the reason they act like that is
arrogance. Stark and Fern are just so beneath them (in their minds), why even bother killing them right away? It's not like they could ever be harmed by such lesser beings...
IIRC, that was the one advantage Frieren told Fern to use. Being underestimated.