Question

Why do aircrafts of different countries have a fixed shape?

Answer 1

who said this that aircrafts of different countries have a fixed shape

Answer 2

An aircraft's structure does not depend on the country which it belongs to. The environmental factors matter.

The principle parts of air ship configuration are

1. aerodynamic

2. propulsion

3. controls

4. mass

5. structure.  

Wing design and fuselage are also an important part in the designing of an aircraft.

So whether an aircraft belongs to India or America the shape will always remain the same