I think it's about money, when there is money involved it gives you adrenaline rush, and gamblers seems to be addicted to it and they can't seem to find that feeling anywhere else.
Yes it is about money and trying a luck, but in the current era games online can also make a good source of income if you are an expert in certain games online that will have a good impact if you have channels that can be seen by many viewers. So in my opinion in the current era anything will have the same reason about money, and if you have skills in related matters then you will have good potential to earn income from whatever you do so far.