Warehousing I suppose is also a factor, your comparing FMCG with Premium bike parts and, yet again these 2 markets diverge in a big way...
A set of Flow rims ( relatively bulky) purchased from a shop will sit in a box in storage and then in a shop or being oggled (but not purchased) for months, all the time effectively being "Inefficiently" stored and incurring costs for the seller...
Warehousing is probably one area where money can be saved on assembly of a complete build wheelset; either by employing a basic JIT stock control system and planning production/supply months in advance around forcast demand, or simply buying in suitable bulk to get bigger discounts and making sure you use every square foot of your warehouse as effectively as possible...
The piece parts you buy from a shop will never come through the most efficient supply chain possible as they basically slow moving, luxury consumer goods hence never the best value...
Basically everything has a cost that will be passed on to you;
Storage, handling, manufacturing, assembly these can be offset sufficiently through bulk Qty that a complete assembled product costs less than the DIY assembly of aftermarket piece parts would do...
