why can't I just jumper the ground and neutral in an outlet?
The lingo we use for doing that is called 'bootleg ground'. As for potentials this exists between all three wires in a 120/240 volt system in residential wiring.
As for tying the neutral to the ground at the receptacle you need to remember that the neutral is the return leg to the center tap of the serving transformer and is therefore a
current carrying wire. The equipment ground is a low impedance path intentionally installed to bond all metal in a branch circuit or feeder and is never to carry current except during a fault event. This is to allow any fault currents from ground faults such as a hot wire coming in contact with a metal box to return to the source
not earth. The source is the transformer powering the residence. Without this low impedance path back to the source you could never get enough current to flow thru the breaker and trip it on fault. So the 'primary' purpose of the equipment ground is human safety not property protection.
Now to explain why we bond the neutral and ground at the service equipment (main disconnect panel). The utility only runs 3 wires to your meter and then to the main disconnect or breaker panel with a main disconnect installed it. The service neutral is connected to the neutral bar/terminal in the enclosure that houses the main disconnect. The service neutral at the other end (source) is connected to the center tap of the transformer. It is the
only low impedance path back to the source for neutral current and fault current line side of the main disconnect panel. Therefore we bond them at the service equipment (main disconnect enclosure) so that both can utilize the service neutral to return to the source.
If we were to bond neutral and ground load side of the main disconnect like at a receptacle we would be giving the neutral current another low impedance path back to the source..ie...it would flow on your equipment ground wires. No current from the operating system is ever wanted on the equipment ground. Remember current will take
all paths given to it to return to the source. Given paths of equal impedance/resistance the current will split evenly on those paths. If in the case of a bootleg and the presence of an equipment ground in your wiring and you would have a neutral open in the receptacle box
all neutral current would transfer to the equipment ground to get back to the source. Not what you want.
Your grounding system in a residence or more appropriately your grounding electrode system (GES) such as metal water pipes and ground rods are there for property protection not human safety. Thing you need to remember is current will not flow to ground (earth) in appreciable amps at low voltage like house voltages because earth (dirt) it is not a good conductor. It offers too much resistance. However in the situations where you have huge voltage events, like lighting or a utility high voltage wire falling on your overhead service to your house, current will flow too earth because it is being pushed by huge voltage. So ground rods are there for this type of event but are only supplemental electrodes. Your water pipe if it is metal plays a much more important role than a ground rod assuming the waters pipe is in contact with the ground and is a continuous metal system with the utilities.
Now these ground rods IMO play a very small role in surge protection devices at equipment. Reason is they simply cut off electrical to the equipment and divert voltage surges to the equipment ground which will take the any current back to the main panel most of which will flow back to the source. So given a good surge protector and voltages it can handle effectively (also remember current plays a big role here) it simply protects the equipment plugged into it by utilizing a
low impedance path to transfer the unwanted surge. If it is simply connected to earth via a ground rod the resistance is so high it likely would not be able to function properly and simply be damaged to the point of failure.