The Wisdom that has tied us to the body is the same that has gone into creation. In fact the very purpose of creation is to manifest the Divine through matter. The human body is therefore meant to be an instrument of the soul.
But first the soul must grow through all the varied experiences while it is in the human body. At this stage the soul continues in the human body as long it is necessary and possible to get the experiences needed for the growth. Even when the surface mind, vital and the ego-based personality has lost interest or have entered a state of coma, yet the inner being is awake and continues to gather the needed experience and thereby helps the soul to grow, even though the body is decrepit and old. That is why euthanasia is never a good thing since its decision is often taken based on the condition of the outer being.
Later when the soul has developed fully then it is free to remain tied to the body or leave it as it wills depending upon its mission and the work it has come to accomplish.
About Savitri | B1C2-10 Condition of the World (p.18)