genetics and environment in short
Its just a function of nature & biolodgy.I get alot from Buddhas writings on the entire human experience .I think he relized the nature of humans better than anyone
I would say that it's because as a whole, nature constantly changes. I certainly wasn't the same person two years ago. We decay and regenerate all the time.
Spiritual growth opportunities. If we were all alike, there would be no challenges in life, and therefore no purpose in living in the physical realm.
To give a better chance for the species as a whole to survive.