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The main reason for the cylinder shape is to reduce the stress that comes from pressurizing an airplane. When you add air pressure to a lightweight, hollow structure, it will try to assume a round shape. This could mean failure if the structure isn't strong enough. A round cross-section distributes pressure evenly, so there's less worry of a crack or break. 

The cylindrical shape is also a very aerodynamic, streamlined shape. When a plane flies through the air, it's going to meet with air resistance, called drag. The cylindrical, teardrop shape allows for the plane to fly through the air with the least resistance possible.