Why the people lose their money because they gamble for a long time and if in middle they win, they will continue to gamble
It doesn't matter, long time, short time, middle time or whatever time, you can be so unlucky that you won't ever win and you can be luckiest guy on planet and win every time, or just regular gambler who will sometimes win and sometimes lose.
at the end they lose all their money because gambling is overall a bad thing
You are wrong, people lose money because they place bets while odds are against them, not because gambling is bad thing.
as much people I know who gamble online or offline they have lost all their savings and much more, because when you cross the limits then it will harmful for you. If you gamble for fun then its ok but dont cross the limits ever.
Obviously those people have gambling problem.
I think it would be a big blunder from ones side if he will think to make more money from gambling than he had at the time because this nature of greediness leads you to hell and on earth, your life becomes at that time when you have to face financial crisis and no one have the stamina to face them for a long time. I think the amount you have won in gambling, thinking of it as a blessing would be good for you.