I would like to pre-empt this post by pointing out that I completely understand why items that have inherent magical properties require the rules as writ in gold cost and time. Incorporating magical elements into crafting of any kind will invariably take longer and cost money in materials or expertise from someone who can tell you "not like that, Idiot, you'll blow us all up."
With that said, I do not understand why mundane items, especially things like simple weapons, require a full 4 days and 100% gold cost for an expert or legendary crafter.
My arguement is three-fold. Time. For a barely trained or master crafter I could understand basic items to take a fair amount of time, however it is horribly impractical that an expert or legendary crafter is incapable of mounting a shaft to a spearhead in a day, even after acquiring raw materials. If you went to a proper weaponsmith as said a spear should take 4 days, they would laugh in your face. If I play a 15th level Dwarven Paladin of Torag, who wants to spread the good word of Torag by crafting defensive weaponry for a town on the verge of being assaulted, it makes no practical sense that it should take me a minimum of 4 days to outfit a single person with a simple weapon. I would have stood better time to make them something absurdly magical far beyond their capabilities to wield and sent them off to fight as a singular hero to the town... who is then immediately overrun by enemies and killed, because a horde of simple weapons would have served a better purpose.
The second part of my arguement, Cost. My Dwarven Paladin of Torag notices the local town has a mineshaft with raw ore to make steel, and lumber for cutting down to make perfect spear shafts... Who am I paying money to for crafting the spear? Getting all the raw material myself I expect to take time, but where do my gold and silver coins go if I have everything I need and the Artisans tools of both woodworking and metalworking? Acquiring materials naturally and using them to create is the hallmark of a professional crafter, as an adventurer my character carries tools with him, and as a player I know exactly how to create without spending a single copper, it seems strange that procuring food is well detailed but procuring special material for crafting isn't even a whisper mentioned in the core rulebook.
The third and final part of this fairly long winded argument, Collaboration. If I know that another character is capable of crafting a portion of a weapon, and I'm capable of creating the other portion, why is collaboration a circumstance bonus instead of a much more functional reduction in time, the way manufacturing works normally? When you attempt to build an item of many complex parts across many craftsmen, you don't wait individually for each piece to be done before another is started, you both work for less time to complete the same amount of work. If my Dwarven Paladin is accompanied by another crafter who would like to aid in my endeavour, they can either craft alone and produce a second piece of equipment in the same time, or... aid me in producing one piece with a circumstance bonus irrelevant to my overwhelming legendary crafting ability, in exactly the same time. A second example, in one of the first adventure paths, players can find themselves in possession of a suitable place for crafting. There is also a fairly deep staircase the leads to an open cavern and a forest nearby. As a player I would like to start a logging and mining group using paid labor to procure raw material and then a team of crafters to produce non-magical weapons to outfit the town and tower. I would understand paying labor, but where am I spending money on making the item, and again, why does my team of crafters take 4 days on a single item? For the sake of story having a team of dwarves singing hymns to Torag as they craft weapons and armor for the local garrison sounds epic, but the rules as writ make those dwarves, and my character, sound like bumbling children that need an absurd amount of time to do the most basic of things.
In essence, for crafting, I feel as if the rules were created for selfish makers to craft only the best of items for themselves and maybe their team or companions, but not much thought was given to characters built around crafting generously by simplifying the process of crafting to an assembly line process for ease of cost and time.