So, to jump on this hot topic, let's do a slightly longer deeper dive on the construction of the Titan today:

1. Carbon fiber reinforced polymer composite has been long used for pressure vessels, for example, the water tank on top of your local water tower is most likely made of fiberglass.
2. The advantage of using carbon fiber is strength to weight ratio. In engineering, the more weight you save, the less extra things you have to do to support this weight, which leads to more weight, to put it simply (which is why factor of safety usually only goes up to 3 for most engineering projects, and for aerospace, it's around 1.2 because every gram counts there)
3. Now here lies the problem, fiber reinforced polymer composites are only strong in the tensile (stretching) direction, and not in the compressive or buckling direction.
4. What that essentially means is that the carbon fiber is good for vessel applications where the INSIDE pressure is higher than the OUTSIDE pressure (most vessels in atmospheric or lower pressure), and not good if the OUTSIDE pressure is higher than the INSIDE (vacuum chamber, submarines, etc.)
5. So why did they design it that way? Cost, it is way cheaper to shape fiber and plastic into the shape you want versus using the industry standard titanium alloy designs, the equipment you need are probably much smaller, and you don't have to hire experienced welders.
6. The other important thing I should mention is that polymer composites are very sensitive to defects in the manufacturing process, as what you are doing is essentially alternating deposits of the fiber cloth and plastic, layer by layer. So, things like trapped micro air bubble, improper adhesion between layers, microcracks from the minor variations in temperature, all of which are very difficult to inspect for, and in such extreme conditions, become the origin of failure. (Which is why force simulations are generally not reliable without real life testing, and you always need to overbuild for safety).