If we're talking about bows, then keep in mind that the actual kind of wood used in creating the best bows was incredibly important. Different woods have different amounts of rigidity, tensile strength, etc. You also had to ensure that the inside of the bow used the more flexible portion of the wood, while the outside had the more rigid, whether by laminating a composite bow together, or by carving the bow's wood so that it takes different portions of the tree into one solid piece.
Again, I think "make a door" is too simple an example - you can imagine a solid door, and file down anything to a big, flat board. Now try to imagine how to build the inner components of the knob of that door, including how the lock works.
You can't really just imagine your way to chemistry, it only comes from experimentation and many, many years of it, at that. Yes, you could eventually do it, theoretically, although it may take generations to relearn the very fine tricks that get passed down. It's no trivial task, however.
Guilds existed in the middle ages because they could control the access to the knowledge and tools of their trade. Stonemasons were valuable people because they were the ones who had training in how to make archways and how to make stone bridges or multi-story buildings that could support their weight. Metalsmiths were valuable people because they knew the intricacies of how to balance the tricky chemistry of the metals they worked with. Farmers, especially, had aquired a great deal of information on how the composition of their soil worked, and how to maintain it, even without having any clue as to why soil fertility worked. A good chunk of this Improved Farming thread I have been talking about involves arguments over NPK, soil pH, CEC, and several other things I had little clue about before I started performing research on the topic.
Farmers didn't know how their soil worked, only how to get it to work, and because of it, were probably some of the most fearful people of change and experimentation in the era, since they had no idea if any change in their farming methods might risk famine come next Winter.
On a totally unrelated note...
If critical fails on crafting rolls wasn't included because Toady didn't want players to waste irreplaceable materials, then why does he let doctors make sutures out of cotton candy? Or making moods consume 5 different pieces of cotton candy for an artifact toy anvil? Cotton candy is pretty much the only material that isn't replacable in this game, and that's why you have to make sure nobody but a Legendary +5 can even touch the stuff. Even things like black diamonds tend to come in the dwarven caravan every year.
I'd much rather have my rank noob stonemason waste some microcline failing to make doors than to have adamantine consumed by being turned into thread. Maybe we could make some kind of trade?