Like Jakebeth, I've got no expertise in this, but I understand that when an airframe reaches a certain number of hours, a check must be done (and with more hours come more stringent checks). I've heard that the "C-Check" is quite intensive, and costs quite a bit to do.

Perhaps the planes are in need of these various checks in order to fly, and the cost to update them isn't worth them being sold to some other company (which would explain so many seemingly "alright" planes being torn up for scrap parts).