As for the question of why we aren't allowed to retain our memories from birth (after all, we're born at a distinct disadvantage with our very weak infant bodies depending on our parents for our very survival, which further subjects us to the potential for injury through various traumas) -- I'm not absolutely sure of the reason.

Except, maybe part of it is that life itself is a test to see whether or not we will wake up to these facts, and take it upon ourselves (both individually and collectively) to wisely ~manage our environments in such a way as to reduce, as much as possible, further traumas to ourselves and future generations. To do so successfully, would require a lot of caring, ~loving attention to the problem. That's an issue, in a society where many individuals of every educational level are unable to even ~define terms like "love", "caring", etc..

So far, as a Civilization we've failed that task, although superficially at least, our "leaders" proclaim to have only our best interests at heart. However, on the whole, judging from the ~results of most of their decisions and policies (all with wideranging effects on the entire population), it's obvious that something about their system(s) isn't working correctly. Because, there's been little improvement and in many cases there's been only more deterioration and destruction of both Human beings and our environment/habitat -- in turn, further robbing people (and even animals, plants) of not only life, but of our very ~memories (thus our sense of identity).

It's much easier to enjoy some measure of success as an individual, or better yet (though harder to achieve), as a family group, than to try to redirect the 'herd' in the right direction. That must be due to the fact that so many people have lost their memories already. They've lost their true Selves, their identities, and either can't or don't want to recoup them. And it must also be due to the fact that ~most people in positions of power, authority, or fame, are (and have been, historically) hopelessly lost and confused.

But what would we be like, if we were never in danger (through traumas) of losing our memories and therefore our Selves? We'd be God-like, I think (in the truest sense of the word). We'd be true children of God, our Creator. Like the healthiest of infants, we would be deliriously happy all the time (or, at least very calm and at peace with ourselves and others). We would worry about very little, because there wouldn't be that much to be concerned about (ie wars, enemies, etc.). We wouldn't live in fear of accidental death either, because we would feel secure in our spiritual knowledge. At the same time, such an existence devoid of dog-eat-dog competition and struggles for basic survival (not to mention greed-based materialism), would reduce the incidence of accidents anyway.

Maybe that's what happened to Adam and Eve, when they ate from the tree of knowledge of "good AND evil" (as opposed to only "good"): they became confused and forgot who they really were. The Ancients went from living forever, to lifespans of 'only' hundreds of years, to what we see now: very short lives.