It's a matter of basic economics - most abandoned aircraft are in remote regions, and the cost of retrieving the scrap metals from those difficult-to-access areas, is more than the value of the metals.
It's rare for abandoned aircraft to retain their engines, because they are high-value items that are usually salvaged, while the low-value hull and wings are left behind - they are regarded as having no economic value.
Aircraft graveyards are usually formed, so components can be used as spares, sometime in the future.