They rarely ever do that in the book. Rather than humans that can turn into cats, seems they identify more as cats that can turn into himans.
That scene in the beginning where they took human form seemed a one-off for them, - they took a form they usually wpuldnt to be avle to communicate without revealing they were cats, in order to warn cale.
Im in the second season of the novel, 273 ch in, and they have taken human form like a handful of times.
Honestly if I could turn into a cat, I would. You get to sleep whenever you want wherever you want. Food. Bed. Brushing. You would be extremely pampered (by a certain butler).
Why does the kitties not transforming back into their human form anymore?