People with jobs I wouldn’t want to do work very hard to bring us these devices, and the earth gives up resources to mine, transport, and shape the materials. Throwing things away when they could last 5-10x as long consumes our precious resources and wastes our collective capital.
Finally I’ll say that a lack of open software support contributes to this. If I could flash alpine linux on an old iPhone, Nexus, or Samsung, I could find great uses for the old hardware. Such a project is now underway (Alpine linux for Mobile), but mobile devices are still a morass of unpatchable secret binaries with glaring security holes.
I know we can better use our resources than to throw a two or three year old flagship phone in the trash or let it waste away in an old drawer, but it will take us demanding more of manufacturers and calling them out for their waste before they take action on it.