I might be wrong here, but I think that quite a lot of car manufacturers actually sell the bearing already fitted into a new hub, reason being, at
dealer hourly rates, its cheaper for the customer to do it this way. There will aways be motor factors that will sell bearing assembles by them selves, but I can't comment on what the end cost is - ie "bearing + removal/refitting to old hub" versus "bearing and hub assembly". A lot of the time the extra risk is that the hubs nowadays are not as over engineered as in the dark distant past, so even slight abuse when removing/fitting the new bearing will crack or damage the hub - so the repair cost spirals up!