Transmission lines in USA, household transformers - efficiency and economy
I can't definitively answer the questions about efficiency, but


  Question 1: Every house has separate transformer?


No. One pole transformer handles about 200 kVA and typically serves about 5 to 10 homes.


  7.2kV overhead transmission lines are present over all streets at small towns and villages?


Yes.


  Seriously?


Yes.

The reasons are probably mostly historical, but there are advantages.

Since the "low" voltage (240 VAC) distribution connections are very short, the I2R losses are low.

The failure of a single transformer affects a smaller number of customers. Repair is very quick.

A lot of the USA is very rural, with a low population density. Smaller transformers are a better fit for those customers.