It doesn't matter how many gamble, what matters is how much they gamble.
And with the US gamling industry being nearly $250 billion, that's close to the entire GDP of Nigeria, basically everyone there would need to gamble it's entire wage to come close to those numbers. Nigeria barely has 3 billion in the gambling industry, it's peanuts compared to the big guys.
Yeah this is the answer for people who wondering why their countries aren't included on the list.
Just check the website posted by @OP, they rank the countries who gamble the most based on the losses instead of the numbers gambler/population ratio.
I was surprised that Japan is on the list. Especially considering how tightly regulated gambling is.
So for me, it's kinda gambling culture doesn't always look the same everywhere, especially in some countries that have famous cities thatare really into gambling like Vegas or Macau.
It's true Japan is really tight, but gambling is popular on there. What they play is
Pachinko, something like pinball arcade games.
I think the reason why many Japanese gamble because if they work under senior who're gambler, they're forced to gamble too in order to respect their seniors.