In the real world, you can sell your old stuff. People just don't.

You can sometimes sell your old stuff but for many people it's not worth it for most items - the return vs. the time investment isn't worth it vs. just throwing it out.

Even giving stuff away for free is generally too much effort to be worth it over throwing stuff out though you might think people who had a use for free stuff would have an interest in making it easier to give it to them than to drop it in the garbage.