Because sorting them on site and just moving the valuable ones is (probably) a lot less work than moving raw ore umpty squillion miles and then landing it softl

Because sorting them on site and just moving the valuable ones is (probably) a lot less work than moving raw ore umpty squillion miles and then landing it softly somewhere it can be processed. Also it means that the things you are throwing in Earth's general direction are comparatively small and not capable of wreaking havoc.