I don't follow the logic.

Because humans are expensive? Or because we can maybe re-use the components if an (expensive) human comes and retrieves the components?

Sorry for being dumb here.

A combination of:

- humans are expensive.

- If you want a custom part, you will need specialized equipment to build that part.

- If you want a custom part, you will maybe need to transport that part all around the world, while the off-the-shelf components might already be available close to your assembly plant.