But the thing is there are gamblers that can't stop playing even if they have won a lot already and think they can make more out of their winnings but end up losing in the end.
You are assuming that they are losing to prove that they lose.
Just because they are wagering does not mean that they are going to lose. Just because they are greedy and continue to bet does not mean that they will lose. And if you are considering only one player, then the variance is high and you will deviate from the mean of 0.
Basically, your argument boils down to this: human emotions cause greed, which means a higher wagered amount, resulting in the loss of money.
This may be true with games that have house edge, but it's not true in the case of zero-edge games. And not only that, but you are also assuming that the casino has enough bankroll to cover that of the player.