i've always wondered the same thing. i would assume that it is hard to buy on the GE, as people sell them in the rs black market section. i also know people use them for cooking pizzas and stuff. some of which will make you a profit. so i can understand where the demand comes from. whether or not thats the actual "big deal" is beyond me.
They took out unlimited stock in shops so items like that that don't have an extremely high stock and are in really high demand are worth more than their trade value, so people either add junk or cash to make up the difference in value.
Cheese has recently become increasingly valuable as new items are added into the game with updates, and the fact that there is no longer unlimited stock in shops. Cheese can now only be obtained in small quantities such as on tables and on the ground in Runescape. It is relatively hard to obtain cheese from shops.