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, y

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.