They will lose a lot of money

and then they wouldn't be able to use sales. The in-game market will inflate, gaming experience becomes dull as prices lower.
By having all these things the in-game economies stay afloat and the devs/licensers get a return on their investment into the game from the diamond sales
Though I do think is prices could use a littttlllee more balancing :3 *cough* plussing jewels