Not odd at all -- when weight is critical, you want to make the most out of every gram of material, ideally getting overall structure, aero skin, and pressure vessel in one.

Rockets with balloon tanks (such as the original Atlas) even rely on pressure from the propellant tank contents to strengthen the structure.

Also automobiles; in the 1960s cars mostly moved from body-on-frame to unibody construction, where the skin is an integral part. The Lincoln Town Car was one of the rare body-on-frame cars, which is why it was often used for stretch limousines; it is much easier to extend the body.

With newer carbon fiber airplanes, there isn't much structure other than the skin.

Yes most if not all modern aircraft are monocoque.