all GPU's rise in price, even reference cards, because in the end, it's all the same to a miner (however a better cooled chip will last longer, but these cards often get underclocked anyway for power savings reasons). For the same reasons you would buy a reference card if nothing else was available, miners do the same. If they're out of Aorus or ROG 1070's, the reference card will still pull in about the same amount of cash, leaving you with none. No stock of a card generally implies demand is outpacing supply. Prices rising has two effects: making the value per dollar less enticing which slows the rate they sell, and allows retailers to cash in extra with absolutely no added cost.
An item is only as valuable as what someone is willing to pay for it, and I promise you those cards have more value to a miner than to us gamers, thus prices will continue to rise until the next tier cards are better value