Then she was a very, very stupid controller.
"Hello, Visser Three. I have a morph capable host, the trust of three more morph capable humans, an Andalite, and a morph capable hawk. This group, which in its entirety, form the 'Andalite Bandits'. Also, I have the Escafil Device in my possession. I would like my Visser status now."
Or, go through the entire series, and end up with your whole society being effectively destroyed in this part of the Galaxy.
It doesn't help that in Megamorphs 4, it is shown that the Ellimist specifically chose Cassie to be in the group due to her positioning in the time stream. The Ellimist would not have had it in his plans for a vital member of his plot on Earth to be a controller.