I can buy a Powerbook G4 for the price of a macbook.
That means PBG4 is indeed very desirable, because they're obsoleted technology and simply old hardware in the first place. In the Windows world, no such thing would be possible!
Macs do keep their value longer. Whether the reason is that they're more desirable or that they just keep working longer, doesn't matter, but we can all see that 2nd hand Apple hardware is more valuable than most other brand PC's.