I can't answer your question like a factbook as this is not my area of expertise. However, I might be able to help partially answer it by trying to highlight some of its assumptions.

First, what do you mean by assembly line? Generally this refers to a process of manufacturing goods in a way that requires the manufacturers to stay in place and to repeatedly do one or a few simple tasks. Each worker thus contributes to a specific part of manufacturing as the goods being completed move down some kind of line that as a whole increases the speed at which the goods are produced (as compared to manufacture by hand).

Well, this existed before the Industrial Revolution for building large structures like ships. What we see are mechanical and temporal improvements to these primitive assembly lines, e.g. with continuous workflows by working in shifts, then the conveyor belt comes along, etc. Yet even if the concept of the assembly line had existed in the medieval era, there were mechanical aspects that couldn't be implemented to make, e.g. a 20th century assembly line. Also, assembly lines actually require varying degrees of training and the industrial workforces had to be redisciplined over many generations (still ongoing, e.g. getting people to follow clocks, not talk or sing while working, only follow unnatural repetitive motions while working etc.).

So it's still a question of precisely what kind of assembly line are you taking about. But you say for something as simple as arrows--the craftsmen would have worked on a part of the good just like in most workshops in Europe then.

Second, you're asking about an assembly as a concept and why the idea of it never occurred and/or was never implemented before the time of your arrows. Well, we have no idea when this concept was first conceived, but we see when it began developing as a thing in the world. Asking why it didn't come into being before it did is akin to asking why something that would fundamentally improve our economy and yet has likely not been conceived by anyone isn't just brought into existence right now. The main reason why this isn't the case for most things of this nature is because invention and technological revolution are usually cumulative processes that do not have one single point of origin. A range of factors such as poor management, conservativism, guild regulations, lack of time for research and experimentation, and so on, may have all hampered such development.

Edit: spelling