If I recall correctly, once Apple has released a laptop product, they are always available for purchase. How come companies like Lenovo cannot do the same?
Lenovo and other PC OEMs don't really have the discipline to stick with one model with minor changes. They're always chasing the bottom line to compete with one another. Even aesthetically identical laptops released within a few months of the other will have slightly different changes; different trackpad manufacturer, different webcam, slightly different screw placement, different charging port...
This means issues with stock as you've got so many different designs and different components. It's an absolute cluster and a reason why a lot of OEM products become EOL or have parts out of stock, even within a year of release in some cases.
Designing a decent chassis out of more expensive rigid materials will, in the long run over 3+ years, pay for itself if you can stick with it. It means the design for motherboard placement and cooling will be standardised. The consumer can still pick the spec of the machine but won't have to wrestle between 6 different models each with their unique pros, cons, and compromises.