Usually, when you say "build a bike" what is meant is to start with a raw frame and do all the work fitting the BB and headset, cutting the steerer, installing all the components, cutting the and fitting cables and housings, cutting the chain, and adjusting everything. This can take awhile.
When the LBS gets a stock bike, generally all they need to do is put the the bars and wheels on and check the adjustments. All the fitting and cabling is done. This should easily take less than an hour.