Why is main's neutral tied to earth?
There are four reasons for grounding the neutral. 

1. Grounding neutral provides a common reference for all things plugged into the power system. That makes connections between devices safe(r).

2. Without a ground, static electricity will build up to the point where arcing will occur in the switchgear causing significant loss in transmitted power, overheating, fires etc.

3. With a floating system it is possible to have a short between both in-house and neighboring systems via the ground path as shown below. Turning a light on in your house can cause a light to go on in your neighbors house too. This characteristic is highly unpredictable.







simulate this circuit &ndash; Schematic created using CircuitLab

4. Finally, by giving ground a return path to neutral, a short to the grounded chassis of an appliance causes a predictable outcome in terms of a fuse or breaker response. This provides a great deal of preemptive protection to the user.

In Summary

In a simple model it appears that not tying ground back to the neutral would be safer. However, in reality, in a distributed power system there is no guarantee of this since you have no way to know if there is some other path back to the transformer via a different route. That is, in point 3 above, you may be in danger of being electrocuted just as much as if your neutral was grounded. 

In the end the other benefits of tying ground back to neutral outweigh the one possible, but unreliable, isolation benefit.



NOTE: From point 4 there is a paradigm shift in the way you need to think about the neutral-ground connection. Do not think of neutral connected to ground, but instead think of ground being connected to the neutral to allow the current from a short to ground to return to the transformer.