I would say that rule is made considering voltage drop. So "as close as posible" does not seem defined completly. Voltaage drop can't be more that 5%.
From service to main panel 2%.
From main panel to branch 3%.
Now I am still an apprentice so I might be wrong, that is why I like to ask questions. Forgive me if I am anoying.
The issue is unrelated to VD in this case. The conductors ahead of the main switch are considered to have no overcurrent protection. You probably have seen fuses on the pole top transformer that supplies the 6 to 10 houses you are all connected to. If one of the ungrounded (hot) wire faults to ground in a service raceway the utility transformer might see that bug bump in current as a load rather than a fault. I have certainly seen a few service panels and raceways burned completely away before the fuse on the transformer or a connection in the meter or triplex opens under the heavy current flow. If this service pipe was inside your walls it might set the house on fire before the fuse on the transformer blows. It is very common for utility transformers loaded to %200 of their rating when connected to residential services. In the case of the panels I have seen burned away I got to the house after the fire trucks.
By encasing the raceway in concrete it is hoped that the concrete will enclose the melting raceway and protect the combustible wall it is inside. Eventually the fuse will open or the raceway will burn away enough to stop conducting fault current.
Then there is the problem of theft from the utility. We have seen more than a few raceways cut open ahead of the meter to tap the utility wires. Pot growers seem to love to do this.
