First, we have to debunk the assumption inherent in that question, which is that neutral does go to ground. It sometimes does, and sometimes does not, depending entirely on where you're working and what you're working on.
The National Electric Code (NFPA 70) requires that neutral and ground be bonded at the main service entrance for residential eletrical service. English translation: the ground bus and neutral bus in your main panel are solidly tied together (typically via a bonding strap). This is done in this and only this location! Bonding ground and neutral again elsewhere in the system will create parallel ground paths, which is very dangerous.
Now, for why the code requires this, we must get into a little bit of what each is designed to do. In a standard 120/240 residential service, each circuit will have 3 wires. A hot, neutral, and ground. Looking at them, it's easy to see that the hot and neutral are the same size and have the same insulation thickness while the ground wire is commonly bare (uninsulated) copper. That's because it is designed to do something different.
The hot wire is the path for current to flow from source (panel) to load (let's just say it's a convenience outlet).
The neutral wire is the return path for the current from the load. Electricity 101: electricity is created by the movement of charged particles, and that movement is because unbalanced electrons sense a path to ground, where they can become balanced again. Things in nature like to be balanced. Without a return path, there's no movement of electrons and thus no electricity. In a single phase branch circuit like our example, the current on the hot wire and neutral should be identical (unless we have some ground leakage, which is beyond the scope of this answer).
The ground wire is a low impendance pathway between things-that-might-become-energized-but-aren't-supposed-to-be (metal housings, copper pipes, steel structure, satellite dishes, and the like) and our grounding rod driven deep into the soil outside by the meter. When you wire in conduit using metal boxes, it's allowable to use the conduit bodies and boxes themselves as the ground. In houses you normally wire with romex and plastic boxes, and so a separate ground wire needs to be brought to each outlet.
So, what is the reason we connect ground and neutral? It's for safety. If you go to your main panel and remove the bond between ground and neutral, you have just created a system with a floating neutral, that is, a neutral that has no reference to earth ground. On the plus side, now you have a system where a ground fault (a specific type of short where the hot wire touches something grounded) does not trip a breaker. Sounds pretty awesome, right?! Except for the fact that in your system, when you get a ground fault, you don't know it. And, everything that is grounded may be energized up to system voltage. So, when you're washing dishes and your cat walks on the wet countertop (the countertop where your metal toaster also sits), zzzzzzaaaapp goes the kitty. Replace "kitty" with "toddler" and you can see why the NFPA went away from this type of grounding (believe it or not, they had a serious discussion about the merits of grounded vs. ungrounded for residential installations once upon a time).
Section 250 of the NEC deals extensively with grounding, and if you ever need to give yourself a headache or to fall asleep quickly, I highly recommend it.
If, by chance, you do read it at some point, you may notice that while the NFPA has all sorts of information about grounding requirements, they say very little about grounding on systems above 240 V. And that's because when you get above 240 V you're getting into engineered systems, where some poor electrical engineer is going to have to stamp design drawings and put his professional life on the line that his grounding design is both safe and effective. And indeed, it's common to see different types of grounding systems in industrial and commercial installations. Ungrounded, resistance grounded, and solidly grounded are all in use; sometimes all three at the same facility!
But for you, ground and neutral are connected at the main service entrance so that when you get a ground fault, you trip a breaker. It's a safety issue, with a minor secondary issue being improperly grounded equipment can be prone to premature failure.
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