Why people in our realm are so fundamentally different? Apparently, almost all of us living on Earth are the results of specific Bardo Thodol events, that led us here. We all failed to get to the higher realms. Except of course those who voluntarily chosen this place to serve and educate. And those for whom this realm is actually a promotion. We were all reborn.

Considering the countless number of realms, wouldn't it make more sense for people to be much similar in nature being born in one place? Does it mean that all the differences (smart/dumb, greedy/generous, kind/evil, materialistic/non-materialistic, spiritual/ignorant, educated/illiterate...etc) are less important, than something fundamental that unifies us? what is it then? Lust? desires? Outside world is the result of one's development and ultimate true nature of the beings here, so we all deserve what we have? Why such gigantic variety and number of variables? to increase the choice selection for the next life?
The truth is, we don't all live in the same world. The world you perceive is a projection of your mind, so what you see largely depends on your energy which in its turn depends on your karma. Phenomenal reality is not an aquarium full of fish, it's more complicated than that. To the extent that our energy is similar, which means we have made many similar choices in the past, our realities will be overlapping and vice versa.

Someone who came here to teach, may not at all live in the same reality that you live in, but they can project an image into your reality.

My teacher said, a murderer lives in a very different reality than his victim, but they overlap around a few attachments they have in common.

It's complicated, but for practical purposes it's enough to know that it all depends on karma (long term) and on your energy/state of mind (in the immediate timeframe). Karma is what we can control by being careful. Energy can be tuned in meditation and also by controlling your attention. Also, Karma affects your future energy, and energy affects your choices, therefore your karma.