Because balance is a function of the basil ganglia, not the cerebral cortex.
But you haven't answered the question of how is it that Terri is able to interact with her eyes, with emotion. She focuses with her eyes. It is not possible to interact as Terri does without an intact cerebral cortex. But all of this aside, if the parents want their daughter, why can't they have her? Even if she were brain dead, by some organ donor group's standards and if the parents wanted her home, feeding tube or not, what about CHOICE? This is a wanted life, damaged, imperfect, but very wanted. Wanted is the catch phrase for the pro choicers... Terri is wanted.
Consciousness is not proven to be a function of the cerebral cortex. Karen Quinlan destroyed that belief. Yet I keep hearing it about Terri.