Actually if you open some protection devices you are going to find a MOV between:
Live - Earth
Live - Neutral
Neutral - Earth
And let's not forget that you consider Neutral and Earth 0V. (For a monophasic connection)
Some resistances you are going to have:
* The Live resistance = low.
* The outlet Neutral -> transformer Neutral resistance = very low ~0
* The Neutral -> Earth resistance = low to very low (below 1 ohm)
* The wires resistances = low
The voltage dropout at Rearth would, indeed, be a very big problem if you had a poor grounding.
And do not forget that MOVs protect against transients surges not a long duration surge. If a long duration surge happens, your MOVs tend to blow up.
For example, to deal with that, some telephone line protection, specially in rural areas, use:
A spark gap -> To deal with voltages higher than 5 kV.
A MOV -> To deal with the remaining HV transients.
Fuses -> To deal with the high current surges.
So a MOV alone is not 100% efficient, you have to use it along with other components, and even with that, often in case of Lightning, nothing can be done.