As Futurama famously said, airplanes are rated for a pressure differential somewhere between zero and 0.7 atmospheres. With the positive pressure being inside the plane.

This submersible needed to survive a differential of ~350 atmospheres. With the positive pressure being outside the craft.

Eh, the pressure gradient is a lot less dramatic in a jet vs. under 3500m of water.

Also the airplane shell has to endure tension forces, as inside has greater pressure than outside, whereas the sub has to endure pressure. Some materials work well under tension but not pressure and vice-versa.