When talking about material choice, one has to consider density, strength, and stiffness (in addition to ease of manufacturing, like weldability, machinability, etc). So for example, the aluminum alloys typically used are about 40% of the density of steel alloys, also around the same ratio of tensile strength. Titanium is somewhere in between. The bending stiffness of a part goes up as a strong power of its diameter (as the 4th power of diameter for a solid rod).
So if a part is dimensionally constrained, or needs fine machining like threads, then it's best made out of steel. But if you can make the part larger in cross-section, you can gain a lot of stiffness, enabling the part to be made out of aluminum and gaining greater stiffness and lower weight compared to steel at the same strength. You can see this in the design of bicycle frames, for example, where larger-diameter frame tubes enable lighter bike frames made out of aluminum or titanium, but the wheel axles, bottom bracket spindles, bolts, spokes, etc are generally still made of steel unless the diameter can be greatly enlarged (eg oversized axles/spindles).
For cameras, there are only certain places in a frame where ultimate tensile strength is really tested, but clearly rigidity matters a lot. This is another reason why wood is still a viable camera body material - wood is pretty stiff for its weight. It has low strength in compression, which is why a wood ladder comes out a lot heavier than an aluminum ladder, but you don't need to stand on a camera. Carbon fiber has some of the same strengths and weaknesses as wood (better suited to cameras or bike frames than to submersibles).
