I'm not a materials scientist, but something has happened to consumer plastics in the last 40-60 years-- it's changed for the worse (I'm assuming in an effort to be "green", which happily coincides with undermining durability).

We used to be able to buy Fisher-Price toys that were tough enough to bear the weight of a car, or withstand multiple generations of abuse by children. I've got plastic toolboxes inherited from my grandfather that still hold up.

Take a look at the ratings and user photos for plastic vehicle ramps on Amazon and consider whether you think you could even get the jack stands in before the ramps collapse. Kids' toys are flimsier than ever, yet despite the "cheaper" construction they're also more expensive than ever. I bought my kids the Barbie Dreamhouse (a $100+ marvel of plastic and cardboard) that collapsed under its own weight. The original Nintendo equipment was light-years more rugged than the endless RMA experience that is the Switch.

We have the technology to make plastics that don't feel "cheap" and the products reusable, but for whatever reasons we've clearly steered away from these goals.

Plastics have actually gotten better in the intervening years, but the advances in machining have allowed far more complicated molds than was once economical. This allows designers to use less plastic, and make it "strong enough". I was recently shopping for a gift for my nephew, and was horrendously disappointed in the Fischer price products- for the money, they didn't seem anywhere near the quality I remembered. However, one only needs to examine something like cordless tools or Lego to realize quality products can be made using plastic.

To be fair, Nokia's Windows Phones used plastic which felt rather valuable and high quality. It was an excellent finish that doesn't feel out of a place in a premium device at all.