Actually, it's not, physical addiction takes place when certain chemicals (µ opioid agonists, such as alcohol, barbiturates, nicotine, etc.) bind to µ opioid receptors. And the chemicals that can be released when playing poker due to excitements are most likely catecholamines (adrenaline, noradrenaline and dopamine) which can only cause psychological dependence. So forget that myth once and for all.
Neither alcohol, barbiturates, or nicotine have any activity at µ receptors, so you are mistaken. Most drugs, like gambling, exert their addictive effects through secondary activation of the mesolimbic pathway and endogenous dopamine release.