Which they want to sell at inflated 'natural diamond's prices, not cheaper 'lab diamond' prices. So their goal is to try and convince people lab diamonds aren't as good or 'don't count'; pretending natural diamonds are lab grown isn't going to help with that. And would probably make the industry look even worse than it already does once they get caught, which they would
I'll also add that diamonds don't expire or go down in price. Storage costs are also very cheap compared to pretty much any other product. Basically, they are extremely cheap to hoard until they sell for the price that the owner wants them to sell for.