Are you sure it's not because their products are literally toys?
Incidentally, there is no shortage of real honest to god tools which are made out of plastics. Glass reinforced plastic can be a very robust material, particularly for its weight, and the weight of a tool is often an important consideration. Fetishizing metal for the sake of metal is often done in the domain of luxury goods where feeling and appearance count for more than objective practical physical properties.
Right now my primary computer is an aluminum macbook. The aluminum certainly looks nice, although I've found that the hard edge hurts my wrists when I rest my hands on it. Would putting a slightly larger radius on that bevel really negatively impact the supposed physical strength of the metal design? Nah. But it would negatively impact the aesthetics of the laptop. Apple made it sharp because sharp looks thin and looking thin sells well.
Contrast this with the T60 I was using a decade ago. It's not thin and it's pretty damn heavy. It's also got a metal skeleton but plastic exterior. But for all the faults of that laptop, its plastic components do not make it fragile. I dropped and stepped on that thinkpad more times than I can even recall but not once did the plastic break. Plastic, the right plastic, is actually pretty damn durable.
The thinkpad of course is still a typical consumer good. But next time you're walking past a construction site, take a look at the tools being used by the workers. There are a lot of plastic parts among those tools, and the tools hold up just fine in an environment far more demanding than anything you put your Apple product through.