Because almost all of the surviving forerunner things were built at the same time by the same people in the same style?
I mean, If you look at a Humvee and you look at an Abrams, you'll notice the difference in style between that and a Mark 1 and a First World War field piece.
Also, it needs to be distinctly recognizable as forerunner, where as human things are built off a massively larger preexisting frame of reference for the various styles of human construction.
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