I would guess firstly it takes an understanding of " Objectionable Current " to be able to understand the explanation. I have attached a nice article from Mike Holt for your enjoyment.
The more serious issue with objectionable currents is down stream where the improper case to neutral connection is made. Since electrons ( we will keep it simple and just use electron theory flow…ok ) are trying to get back to the source…if you provide additional paths down stream of the service disconnect then you have electrons flowing on all available paths " proportional " to the impredance of the path back to the source…this can cause some dangerous effects if you have poor connections, personal contact and so on…and what happens IF you main path back is broken…?
Making the connection at the service disconnection means is acceptable in the NEC because it the source of which allows the electrons to flow back to the source which is at the transformer itself…not sure how you would do it any other way unless the power company dropped you down a 4 wire drop…all the time and they are not going to do it…simply economics I guess…
We have come a long way in removing these remote case to neutral situations which would allow multiple paths back to the source but once you get to the main service disconnect we have no choice in the matter…so why bonding is so important to create a low impredance path back to the source…nothing better than a main bonding jumper…so if I did not answer your question I can only say…because the NEC allows it…simply because at this time their is no other choice unless we change the way the POCO does business…and that is not going to happen.
Hey another great article by Mike Holt on the subject…again since the case to neutral connection at the service disconnect is required by the NEC because without it you just may have OCPD’s not able to trip and since the connection is made at the service disconnect we have a ensured low impedance path from the buss bar to the grounded conductor BACK to the source which is at the transformer and it removes any remote case to neutral issues that create the direct hazzards of the improper case to neutral connections we are used to seeing.