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The story of Adam and Eve is not literally true, it was meant to be allegorical, Adam and Eve representing all men and women. God created the physical universe (heaven and earth, i.e. everything that -is-) for a purpose, as part of a big complicated plan of some kind. Of course if that's true, the plan must be so complicated that we would have no chance of understanding it. Like an individual yeast critter being grown to make bread--the individual yeasties have no understanding of what they're doing, they're just floating in dough, eating sugar, burping out CO2. Life is good for them! Until we turn the oven on.
The physical world, and our bodies, were designed to be imperfect, to be temporary. In fact the whole idea of time itself is part of the physical world. God and the angels are in -eternity-, where all times are the same time. We are trapped in time, we have to live every second as it comes, and then go on to the next second. It's just part of life in the physical world.
So the idea is that we each get one life here, and then we go on to The Next Thing (though, understandably, we're a little fuzzy on what that is, exactly). We are like 'prototypes' of angels. We have problems and pains and sorrows that the angels don't have, but OTOH we have great pleasures too. Angels can't enjoy the taste of chocolate melting on their tongue, or being kissed, or riding a motorcycle, or sinking into a hot bath. Somehow a few decades here in the physical world prepares us for life in eternity.
So to ask why are here, I mean why -here-, it's like one kindergartner turning to another and saying 'Why are we -here-?' And the other says 'Well this is just the beginning. Someday you're going to graduate from college and be a famous architect! But first you have to learn your colors.'