By contrast, consider a modern smartphone. There is very little volume to be won by shipping it as parts, and there is greatly increased risk of customer dissatisfaction due to customers-at-large being clumsy, untrained, unequipped and unaccustomed to the assembly task. Just look at how popular it is to complain about assembling Ikea furniture, compare/contrast with e.g. PC assembly, and then scale that up...
What does cost more is final testing, to make sure all the components work together. It is cheaper to do that well at the end of the assembly process before starting packaging, than to deal with a high rate of returns. The testing done throughout the manufacturing process is a significant cost.