>>713066994
Because she's the dragon.
She was supposed to be stopped in Chapter 1, either by being frozen or just convinced it was all a dream.
This is how I think the timetable would look like:
>Susie goes looking for chalk(more to eat)
>ends up in the closet
>Alphys find it strange that she's TAKING TOO LONG
>asks Kris to go look for her
>Noelle asks to tag along
>ends up in the closet too
>since Susie is still the bully, she practically went ahead and started axing people left and right and is on the way to the castle
>everyone terrified, asking for help and/or engaging fight or flight to Kris, Noelle and Ralsei
The rest is story, on what I commented earlier.
I'm basing this partially on shadow crystal visions.