May be because Paravti had to be born twice to unite with Shivji. She was seperated once from Ardhanarishwar form, then as Sati. She is the active energy in all living beings, so Moha or being aasakta is just one of her nature of Shakti. If she too becomes anasakta like Shivji then they wouldn't have united, there would have been no creation to begin with. She is with him after may sacrifices,seperations, tough penance to get himback as her husband. This is part of the Leela, going on in the World. May be to tell Shivji see how strongly your devotee needs you, you are needed here too.
Even Shivji balances both his devotees & Parvati's sides by giving each proper attention. After attending Ravan he has to be back at Kailash. So being anasakta he is solving both the aasakti, one of his devotee's for him, who wants to have his darshan desperately & other Paravti's who needs her Husband.