The problem with the gems is that Anet’s prices are A LOT higher than the farmers when you compare Anet gem purchasing to buying gold from a farmer. Thus, nobody in their right mind would turn gems into gold. Last I did the math you spend around 500% more money through Anet compared to a farmer.
Therefore, we have a situation where NOBODY turns gems into gold, but EVERYBODY is turning gold into gems, causing the gold —> gem rate to skyrocket, until it equalizes at around the farmer prices.