Its very simple like you say - but its difficult to explain to the addicted gamblers who only think of their wins and cant stop even after they win a big because their greed wants more - and that greed s never satisfied. It is possible to control your greed and not gamble like crazy. Then it remains a mode of entertainment by paying the casino owners - in that way.
Almost everyone wont stop when they are on winning streak. If you managed to turn $100 into $200 then probably you wont stop right there until you lose few of your profit. Some people are smart enough to stop when they suffer small lose while some will keep on going until they could keep on winning again
Most people are not smart and that is the reason why casinos are a flourishing business in countries where gambling is legal. The fact that one can turn their money from dust into millions makes then play all day and night and the net profit is there for the owners and not the players. They keep chasing losses and the casino profits. That is how the casinos milk their players.