Same with Mini, you can purchase one from stock if you want, but most people want to spec their own car. Most parts on most bikes seem to be bought in.
It is more complicated to administer and takes a higher degree of organisation by the manufacturer and the suppliers, but based on my experience anyone offering it would pick up plenty of extra market share in addition to raising the average price of their final orders.
But I agree that it will probably never happen and that there will still be a brisk trade on eBay for the discarded bits as purchases upgrade!
.. variance costs .. colleagues of mine from Germany once told me that every vw golf was unique or at most two identical. When you factor in body colour, upholstery, wheel trim , motor type, fuel ., power, wheel size, audio system, etc etc. . However that means having a sales agent talking through options , and entering data into a database. It also means having multiple inventory, all costing money. In the case of VW the perceived value in having a unique vehicle to the German consumer was worth the extra cost. They have automated production lines with computerised inventory, and waiting lists of up to 6 months. That is the cost of variance.
The bicycle construction business might be as automated, but who is going to talk the consumer through the options... And who will pay them.