Why does this happen? I think a partial explanation is in this Nietzsche essay [1], where he says, "every concept arises from the equation of unequal things" — in other words, the abstractions of the world were built bottom up in our heads, and the Platonic essence of things is just a fairy tale we tell young programmers so they can sleep easily at night.
1. http://nietzsche.holtof.com/Nietzsche_various/on_truth_and_l...