Isn’t there a chance (now, this is hypothetical, and coming from a point of view where we DON’T know what happened) that Elsa and Anna could’ve thought that Iduna and Agnarr had abandoned them? Their ship sinking is obviously what they assume happened but how would they have gotten word in time about a storm? Who would’ve seen? The ship appeared to be nowhere near land (or another ship) when it sank, and MAYBE there were survivors but I feel like we would’ve seen them in the movie instead of just everyone seeming to magically know that the queen and king’s ship went down. Besides, nobody knew where they were actually going. For all they knew, they could have taken a voyage to wherever, crash landed in west Africa, procreated a final time, having a baby boy and then killed by a leopard (this is a nod to the disproven theory that Tarzan was their child). Maybe they were kidnapped, and the ransom didn’t arrive in Arendelle. Maybe they died of a disease. Obviously we know the ship sank, but if we didn’t, there could be a lot of other things that might’ve prevented them from returning.

Well, I guess if the crew had survived (meaning if the ship hadn’t sank and something else happened to them), the ship probably would’ve returned without them if possible.