One reason surely is that it's Japan's one legal outlet for gambling, allowing players to try their luck and win money playing pachinko. In fact, during Japan's long economic downturn many turned to pachinko in a last ditch effort to make ends meet.
Crime. Gambling is illegal in Japan, but pachinko is regarded as an exception and treated as an amusement activity. Although awarding direct money prizes for it is illegal, parlors may reward players with tokens which can then be sold for cash at nearby exchange centers.