The fact that we may not remember our former lives does not mean that we did not exist before.
Or maybe what was before is not the same as it is now.
Reincarnation is the only logical solution to the mystery of the purpose of our lives.
Only God knows that. We cannot assume we know "why". That is illogical.
I think mccoy is on track, we change all the time even in our own lifetime. We don't really know what kind of state we were in before. It may be something completely different than Earth. If we think we were here before, or if some vision has shown you past history, no one knows if those are there to divert you or not. If you're susceptible to such things, that would be an ideal doorway for diversion.
I think we always existed and always will. Trying to put that into some sort of physical theory is plausible, but not the be all.