because they usually have slotted dropouts, such 10s bikes make viable singlespeeds, which are worth some money rather than nearly none, even when they are tatty. You can convert such a machine for the price of a 3/32" freewheel plus your time to redish the rear wheel and strip off the derailleurs (which can be sold separately).
One day almost any bike that is unused and of a certain age may be worth money, but that day has not yet come for bikes of this sort....
cheers