Because it's fast and easy. Take a big sheet of metal, cut it diagonally in half, and you get two triangular wings. Won't win you any recognition awards, but it'll look like an airplane. Which you can then blow up! Range targets are the only likely things I can think of at the moment. They wouldn't be building or modding drones right out in the open for an extended period, and as the fuselages have been there for a while waiting for use working on them exposed like this seems to indicate that they aren't too sophisticated.
With all of the drones there, Dingxin is probably the launch point for targets for the Shuangchengzi SAM test range located to the north as well.