Consider the fuselage of an A300. I have noticed this not only with airliners, but also for private jets and turboprops: why is the fuselage circular-shaped? Why don't aircraft have square-shaped fuselage? Does it have to do with too much drag?
If you pressurize any hollow structure, it will try to assume a round shape. If you want to create a lightweight pressure vessel, again a sphere will be the most efficient result, because there the stresses in the skin will be equal at every point. Blow up a party balloon if you are in doubt.
A sphere is not the most efficient shape for aerodynamics, so fuselages are elongated spheres with a nice fairing at the end. If you make them cylindrical in the middle, you can build most bulkheads on the same jig and can interchange sections of the internal fairings. Plus, if you need a longer or shorter fuselage for the next version of the plane, you can easily add or remove sections - the parts will still fit after the modification.
Also, some fuselages are combinations of cylinders. The Boeing 377 Stratocruiser used the lower fuselage of the B-29 and had an upper, larger cylinder on top to give the passengers more room.
Boeing 377 cross section (picture source)
For the same reason, rocket stages are cylindrical, too. They also need to be aerodynamic, have to tolerate high internal pressure and need to be lightweight.
The fuselages are circular (or nearly circular) in shape for two main reasons:
The main reason is that for a circular cross section, the pressure loads are resisted by tension, rather than by bending loads in non-circular sections. Also, the non-circular sections have stress concentrations when pressurized, which may lead to failure.
In case of a circular design, the flow will not separate under small (to moderate) angles of attack and in sideslip.
In case of non-pressurized aircraft, the fuselages are dictated by volume constraints and are usually rectangular in shape as it is more efficient in space utilization.
In the case of pressurized aircraft, the best option structurally is to have a circular fuselage, but in order to have a useful internal space, an elliptical or 'double bubble' design is used, with an outer circular section and divided internal sections, like the A380.
Why don't [airliners] have square shaped fuselage?
Shorts Skyvan photo from Wings over Europe
Shorts Skyvan diagram from A Tall Guy
Most airliners are pressurized. If you inflate a rubber balloon you'll notice that the most economical and strongest shape for a pressurized container is one with a circular cross-section.
You'll also notice, when you inflate an Origami water bomb (you should stop reading this and make an origami water bomb now), that the flat sides buckle and bulge - flat sheets are not good at resisting pressure.
Spherical airliners would have too many disadvantages but cylindrical ones have a good balance between strength, weight, drag and space-efficiency.
As you can see above, in some circumstances, aircraft manufacturers do beleieve that the advantages of a rectangular cross-section are worthwhile.
Drag has little to nothing to do with it.
The primary reason why the fuselage is circular (or elliptical) shaped is that the cabin is pressurised.
This means that, mostly during cruise, the interior of the fuselage has an higher pressure than the outside atmosphere. The circular (or roughly circular) shape allow the fuselage to avoid blowing up like a baloon with minimal amount of material, minimizing the overall weight of the aircraft. A square fuselage would break apart at the corners due to stress concentration. A circular shape do not have corners were the tensile stress can concentrate.