Why is the A380 a double-decker rather than a longer airplane?
That's right, having a longer plane would reduce drag by lowering the cross section (altho there is also drag linked to the surface area of the plane).

There is a few reasons why you don't want to do it:


Cylinders don't scale this way:


Take a cylinder with a radius of 1 and a height of 10. 

Volume is ≈31.41

Double the height, volume is ≈62.83

Double the radius, volume is ≈125.66

Increasing the length scale linearly and increasing the volume scales at π * n * n 


You will need a stronger structure to support the additional length and surface area of your pressure vessel, oscillations during turbulences, takeoff rotation, ...
All your cables, hydraulics, fuselage, will have to be stretched 
Passengers will take forever to reach the back of the plane when boarding
Maneuvering in the airport will be impossible, you will need new airports  (credit apple maps)