I think because you can earn money easily thru gambling and some people enjoy the challenge or competition and others just find it fun. Just think of the cons of gambling and the pros of quitting or remember how bad it feels to lose everytime you want to try it again. It might help you to quit or lessen your addiction.
Gambling becomes an addiction for many people due to several different reasons some of them find it as a way to spend their leisure time having fun in the way they want others fall prey to the addiction just for the sake of money and they are hence considered as profit seeking Gamblers.
But this sort of behavior obviously earns you nothing in the end despite more losses. So all I can say is that we must never fall prey to addiction of gambling.