Golems and other constructs have a long history in Dungeons & Dragons and its spinoffs, but, almost invariably, it's extremely difficult for PCs to create them, and they often have varying requirements that don't seem to follow any logical progression, such as the iron cobra in Pathfinder.

This is a construct that requires (according to the Pathfinder Bestiary) 1000gp in materials for its normal iron variant, despite iron only costing a few silver per pound, and several constructs in multiple editions costing gold even when you're constructing them yourself, far beyond living expenses or materials.

Is there a reason for this difficulty? Perhaps buried in the game's history or lore?