> A part sold by itself needs none of those things.
A part sold by itself needs so many more things!
Where do you store it? How do you organize it there? How do you keep track of your inventory? How do you re-order new inventory? How much do you re-order? How long do you typically have to store? How do you source substitute suppliers when your original goes out of business? How do you QC stock? Etc. Etc.
All of that for multiples of the part count that goes into a single model, because you sell more than one model.
You can buy a $3 LED from China (see sibling comment for economics on free shipping) because you're buying the current model / whatever version they feel like giving you.
"Part specs may be subject to variation" doesn't work so well with more complex systems.