It's all because the casino knows how to win and any strategy we use will be useless unless we are lucky.
So there is truth in what you say that the best choice to win is to gamble for fun.
good amount of money and luck should be on your side to get some winnings. martingale or any other strategy applied can't guarantee you will go home with your winnings. always remember that this is gambling and there's always house edge. the dilemma of most gamblers using strategy like martingale is that even if they already got their winnings, they are not yet contented and keeps on playing hoping they will hit bigger amount. however, we all know the end game if you continue playing, your bankroll most likely will run out and there's no going back and say, i should have stopped a lot sooner.