Thing is, the Metal pole is really out of place because it is the only vaguely raider-like decoration available.
And collecting skeleton pieces is possible, but tedious. They're in many many pieces, can weigh a fair amount, and ultimately are physics items, making them a bitch to arrange and likely to be kicked about by NPCs or glitch out when next to/on other settlement items.
Plus, displaying the shattered remains of a person who died 200 years ago isn't the same as hanging the corpse of a man you killed yesterday afternoon.