why can't we sell/trade rewards?...what is the reasoning behind that exactly?

I don't understand it...why would they do such a thing?|||Definitely they are trying to put a stop to the farming. Some people would hoard cards, move to another account, complete a collection again and then sell the reward card for some big coins.|||If we cant sell these reward cards hopefully they will go into a sweet collection later with a nice coin boost and a really awesome card???