I think that you might be onto something with "simplifying something we can't understand".

I read something once, trying to remember where, that made an attempt to explain the nature of human spirit and heaven and basically the physics of the afterlife. The author tried to explain that once we 'pass' we find ourselves in a place that could be described as the 4th dimension. Reality as we know it is kinda represented there... but more so. It's like a square is kinda like a cube.... but more so. We may find that our spirit is merely one facet of existence in the afterlife/Heaven etc. Just as a square is just one side of a cube.

The author went on to explain that this could be why God is represented as a Holy Trinity... because he is more than just a single entity.... one facet is his Heavenly presence, another is his presence within us, another was in human form (Jesus). So... getting back to your question: I think that the reason spirits take on human form is because the person's humanity is a facet of it's continual existence and that we, as mortals are no more able to see the other facets of it's existence as a stick man draw on a page could perceive a 3D drawing.

I used to watch this show called Cosmos in the late 80's when i was studying physics. The narrator gave this example; Imagine a dot, drawn on a piece of paper in a community of dots, suddenly, by some miracle, being lifted up into the air so that he could see all his fellow dots. Then place him back down and see what he says to his fellow dots.... well, it's wont be much. He doesn't have the word for "up".

I think, in seeing spirits (if that is even possible) then we lack the language to describe them accurately or even really know what we are seeing half the time. So our perception is clumsy. They are of another dimension. We don't have the language... we do our best.