Most simply because with the barrel in the way you can't get any pressure on the sand paper. If you use the barrel to push down on the sand paper you have to get the pressure perfect and u now have resistance to the paper moving, both above and below the paper. Wrapping the paper around ever larger sockets is way easier. Put the action in, draw the lines of the barrel on the stock and work from there.
The other issue you have to deal with is the action goes into its opening and then the barrel will sit at an angle with the muzzle much higher than the chamber, just due to the overall configuration of a barrelled action.
