Why isn't there a voltage drop across the Main Wires?
There is a voltage drop. Part of designing a distribution network is to ensure the voltage drop is within a tolerable range, at the maximum expected load.



Let's consider a simplified example.

Suppose that your house power supply is 240 VAC nominal, delivered over a 10mm² two-core cable (single phase power supply.) Your maximum demand is 20 amps. We want no more than 5% voltage drop. What is the maximum length of the supply cable?

Looking up Australian Standard AS3008.1.1:2009 Electrical Installations - Selection of Cables we find some useful tables giving the resistance of various kinds of cable.

Table 34 tells us that 10mm² conductor has a resistance of 2.23 Ω/km at 75° C.

The voltage drop on the cable is given by: 20 amps × 2 ways × 2.23 Ω/km × length (km). The permissible voltage drop is 5% of 240 VAC = 12 V. We can work out that the maximum cable length is 135 metres.



To answer the second part of your question - "how are all the houses on the same transformer getting the same voltage" - the houses are all getting slightly different voltages. However, so long as they are all getting a voltage "close enough" to the nominal voltage, there is no problem.