Why are golem construction rules so complex/confusing/vague/obfuscated in various editions of D&D?

For instance, the iron cobra costs 1000g for the metal to make it. In some places it says it's magically treated, but the price of iron is like 2 silver for a pound if it, though I don't understand why that's required when no other magic items need to be "treated". Next, while I understand the cost if you don't make it yourself, why does one still exist if you craft it? for instance, the normal iron cobra costs 5000g to be made, however, why is the cost only halved (2500g) if you roll to make it. Are you paying the construct itself? Logically this should be free shouldn't it? It doesn't make sense.

Similarly, while they cover turning golems into (nerfed) shield guardians in pathfinder, these golems still require huge fees, and in 5e the building of golems basically isn't explained at all, just a time and gold requirement. Shield Guardians have 0 explanation. What is the reasoning behind this?