Perhaps, they may have heard to some of their friend in gambling you will become rich, that's the thing comes in their mind which leads them to become greed in the money, but unknowingly their interest was already there for them to become addicted in the games while doing gamble in the gambling sites.
Nope that is not the reason why they are addicted. That is the reason why they are attracted to gambling but being attracted and addicted are two different thing. Addicted to gambling are caused by the the hormone produced in the brain, that makes the person rely on the substance to alter their mood.