It's simple:
It's because your soul-memory is temporarily wiped each time you incarnate on earth. (You get your soul-memory back once you return to the spirit-world.)
I've even experienced this "soul-memory-wipe" first-hand, during 1 of my own, spiritual experiences (which was a type of near-death-experience, even though my body wasn't actually dying). At the end of this spiritual experience of mine, I could feel most of my memory of it, being removed from me, right at the moment when I passed through the "spiritual border" and got back into my physical body. Before reaching that "spiritual border", I thought I'd never forget anything of what I'd just experienced. But right after having passed through that "spiritual border", I was left with only remembering about 50% of the spiritual experience that I had just had. So, the "soul-memory-wipe" is definitely a real thing.
The spiritual reasons behind the "soul-memory-wipe":
The "soul-memory-wipe" is performed in order to ensure both that:
- Life on earth is able to provide a highly challenging experience for souls.
- Souls' traumas from past, physical lives, won't significantly, negatively affect their experiences and actions in their current, physical lives.
- All the species on earth (including the human species) will be selfish, cruel and competitive enough to be able to survive.
You see, earth can only be a highly challenging place for souls, if they feel spiritually separated/disconnected from each other, so that they thereby are comfortable with acting very selfishly and being very cruel to each other.
If all the incarnated souls on earth, had their full soul-memory intact during their life on earth; then there would be no true selfishness, and no true cruelty, in this world - and even the survival of the species themselves would be at risk.
And there would be no point for souls to even incarnate on earth, if earth was just another version of "Heaven" - as souls already have a "Heaven", back home in the spirit-world; and souls already have other, peaceful planets to incarnate on, if they truly want to experience a peaceful, physical life.
In short:
Earth primarily exist for the purpose of offering souls incredible challenges. And the only way for earth to be a place that offers souls incredible challenges, is for souls to have very selfish and cruel tendencies during their life on earth. And the only way to ensure that souls will have very selfish and cruel tendencies during their life on earth, is by temporarily removing their soul-memory - so that their altruism (their empathy and compassion towards each other) thereby is significantly reduced, during their life on earth.
In other words:
The "soul-memory-wipe" is simply a necessity, in order to make earth function as intended (namely as a very hard school for souls - which is what earth is actually known as, in the spirit-world).