If the neutral & ground were tied together somewhere on the interior of a facility, the grounding system would then carry a portion of the neutral current and a few problems would arise. For one, an unsuspecting person may open up a ground connection, become in series with the open connection, allowing the current to flow through him. This could be fatal. Another thing is that a loose connection anywhere on a raceway that serves an EGC would arc and potentially cause a fire. The safest place to bond is at the service equipment. Making this connection is necessary to allow ground fault current to flow back to the utility transformer with a low enough impedance to either operate the primary fuses or to allow the fault to burn clear with minimal arcing.
