Because that which is infinite and unconditioned can have no form, and cannot be a being, not in any Eastern philosophy worthy of the name, at any rate. An "entity" is immortal, but is so only in its ultimate essence, not in its individual form. When at the last point of its cycle, it is absorbed into its primordial nature; and it becomes spirit, when it loses its name of Entity. Its immortality as a form is limited only to its life-cycle or the Mahamanvantara; after which it is one and identical with the Universal Spirit, and no longer a separate Entity. As to the personal Soul -- by which we mean the spark of consciousness that preserves in the Spiritual Ego the idea of the personal "I" of the last incarnation -- this lasts, as a separate distinct recollection, only throughout the Devachanic period; after which time it is added to the series of other innumerable incarnations of the Ego, like the remembrance in our memory of one of a series of days, at the end of a year. Will you bind the infinitude you claim for your God to finite conditions?
That alone which is indissolubly cemented by Atma (i.e., Buddhi-Manas) is immortal. The soul of man (i.e., of the personality) per se is neither immortal, eternal nor divine. Says the Zohar (vol. iii., p.616), "the soul, when sent to this earth, puts on an earthly garment, to preserve herself here, so she receives above a shining garment, in order to be able to look without injury into the mirror, whose light proceeds from the Lord of Light." Moreover, the Zohar teaches that the soul cannot reach the abode of bliss, unless she has received the "holy kiss," or the reunion of the soul with the substance from which she emanated -- spirit. All souls are dual, and, while the latter is a feminine principle, the spirit is masculine. While imprisoned in body, man is a trinity, unless his pollution is such as to have caused his divorce from the spirit. "Woe to the soul which prefers to her divine husband (spirit) the earthly wedlock with her terrestrial body," records a text of the Book of the Keys, a Hermetic work. Woe indeed, for nothing will remain of that personality to be recorded on the imperishable tablets of the Ego's memory.
Source: 📚 The Key to Theosophy by HPB.