The market has decided.
Case 1: it was a perfectly functioning computer but the owner wanted something new. They could hand it down, donate it directly to a charity, or sell it through a company like Gazelle.
I don't think I've gotten rid of a single computer in 10 years. I have 4 pre 2009 computers that I know are still in good use after installing a fresh copy of Windows 7 on them (thanks to an MSDN license). One of those four is a 2006 era Core Duo Mac Mini. The 2008 era Core 2 Duo is being used now as a Plex Server.
Case 2: they were returned to Apple under warranty, Apple cleaned them up and sold them refurbished. Apple has been selling refurbished devices for over 10 years.
Case 3: they weren't worth selling or refurbishing.