Not sure if there is some other valid game play reason to have this off by default. If there is I am curious as what it is.
There can be many possible reasons. Probably the most common is that you are pre-building components for a specific ship class, and don't want them used by orders for other classes that use the same parts. Another good use case is to stockpile components if you anticipate a need to build many ships quickly in the future, but you don't want to overbuild right now and exceed your fleet maintenance capacity (or fuel reserves, crew numbers, commanders, etc.). For example, such an emergency build program would be used to quickly replace losses suffered after the sudden onset of aggressive negotiations.
Some people will also use component stockpiles as a way to represent a "mothball" mechanic, in which case those components represent specific ship classes placed into deep storage to be "reactivated" (i.e., rebuilt) in case of future need. In that case for roleplay reasons you wouldn't want those components to be used by some other ship as then you have a Mothballs of Theseus problem.