Answer: It is so that we can gradually correct ourselves. You cannot do it all at once. Therefore, we go through many life cycles.
Moreover, each time the soul goes through what is called genesis or reincarnation, there is a partial reunion of souls, followed by another separation, and again some subsequent combination. As a result, all of the souls are completely corrected.
I advise you to read materials about the cycle of the souls. Souls means desires that are gradually transformed from the desire for oneself to the desire for the sake of the Creator.
From KabTV’s “Fundamentals of Kabbalah,” 3/8/20