People ask for unattainable Pokemon on the GTS for one reason:
There is a cloning method in which you replace the game's save file with that from before you put the Pokemon on GTS. This is done by inserting the SD card into yoir computer and saving the Pokemon data to your desktop and then replacing your save file with this one after putting the Pokemon on the GTS. Then you take the Pokemon off the GTS (it is still out there even though you replaced the save file) and you have two. People ask for Pokemon people would never want to give them or can't give them to make sure no one takes the Pokemon they are trying to clone.
Hope this explains.
SOURCE: Former knowledge