It shouldn't be a surprise, but it is. It's one thing to know that the world will end some day and another thing entirely for that time to be now. But the signs are there, the end times are here and it's all about to go wrong.
On the other hand knowing that something is going to happen allows some measure of control, and the Æsir are going to fight and struggle the whole way. Unfortunately Muspelheim's forces also sense the end coming and are attacking Asgard with greater zeal than ever. The gods of Asgard find themselves too busy to walk among the doomed world to intervene.
So, as they've been doing ever since the Titans first escaped, they have again tapped their young to do in the world what they haven't the time to do.