Assuming the identity of a weaker and younger form was something that would usually put her at a disadvantage.
When confronting a fully grown adult who was very dangerous and known to kill others, many would choose an approach that would protect themselves the most and would ensure success in a way that would result in fewer casualties.
The very idea of sending in a child or using such a defenceless form against such a person like Jack would seem a little reckless.
There were so many things that could have gone wrong from the moment that Ash had knocked on his door.
It was possible that Jack may have decided to attack or subdue her the moment that she was close enough to him to allow such a thing.
The only thing protecting her was being outside the door, but the moment that she stepped inside, she was in a lot of danger.
