Beacause most people are greedy and they are always looking for easy money.
When they win they want more and keep playing but they forget that sooner or later their luck will be over.When they lose they can't accept that loss is a part of the game and continue untill they get their money back or lose everything.
there thousands reasons could explained why gambling so addictive , but the most popular are because people love their adrenaline to get pushed it's make them feel self satisfaction , make them happy. and yeah sometimes it's addicting due they feel how easy making money though gambling , you don't need to work for hours just roll the dice .