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.
But you can put money also while playing Online games?and remember the Competition of online gaming now has a Huge amount of money Pot so I guess OP has a point on asking this
Because in gaming we can totally enjoy to the max and can also gain winnings at the same time