In economic theory, zero sum games refer to activities in which the net is fixed and any gain for one party means a diminution for another. In medieval times agricultural yields were fixed so if the lord of the manor took more grain from his land, the peasants ate less. If your grandson takes a larger piece of birthday cake, there is less cake for your granddaughter (and you). Zero sum games makes us all adversaries. Whatever benefits you must be to my disadvantage as if you get more I get less. One of the remarkable realities of modern capitalism is that when investment is combined with technology and productivity then games become non zero sum. Outputs can be increased and productivity and capital investment allows everyone to get more at the same time. The economic pie grows.
For many years stamp collecting was a zero sum game. The supply of fine stamps was fixed and if you had a certain stamp there was one less for me to have. The vast majority of collectors chased the same stamps. Such a situation was not conducive to camaraderie and is one of the reasons I think that most collectors tend to collect in isolation. Sportsmen have teammates; stamp collectors have competitors. About fifty years ago, perhaps in response to this problem, collecting specialties began to dramatically expand. In the US field alone, prior to 1960, nearly everyone was chasing the same two hundred scarce US stamps. Then plate blocks and FDCs and various arcane postal history specialties began to attain great popularity all of which has expanded the reach of the philatelic game and helped our hobby grow to its greatest popularity in the 1970’s and 1980’s. There were and always will be competitive collectors who enjoying owning material that closes out other collectors from having it. But most collectors prefer a more congenial hobby that increased specialization has allowed.