Probably because they just want a new GPU and it is what they can afford. The GTX 1070 would be the better card, and their used price is pretty good right now. If you want to take your chances with a used card then a GTX 1070 would be a good choice. Personally, I would choose EVGA or Asus that can still be RMA'd with the second-hand warranty (3 years) from the manufacture date. Since the 1070 was released 2 1/2 years ago, all of them should still be in warranty. I have known both brands to acknowledge second-hand owners on the manufacturer warranty.
*EVGA would probably be more willing on the warranty since you can actually see the second-hand owner policy on their website.