I would like to rephrase and expand on Chris's comment a bit.

Like most hobbies, what we collect has no intrinsic value. Unlike most hobbies, what we collect is, quite literally, garbage. At one point in time someone thought these cans were trash, threw them away and often buried them to get them out of sight. Any value placed on "the supply" is not only manufactured and artificial, but limited to an incredibly small number of aging individuals...."the demand". Once the demand goes, there will be a disproportionate supply, and that curve has a slippery slope right back to.....garbage.

Rand