It is because Apple wants to maintain their above average industry profit margins on as many things as possible.
However, the main reason to avoid Apple's trade-in program is not the low values they assign to your hardware, it is because the program is horribly run.
In my case, they claimed the backlight on my iMac wasn't working and adjusted its value to $0. They only offered to recycle the machine for me. So, I requested it be sent back and what do you know, the iMac was in perfect working condition. My guess is that had too many iMacs returned at that time decided they weren't going to honor their quotes. Just do a search and you will find many stories where the value they agreed to pay is substantially reduced because of nonexistent problems with the hardware.
-kp