because gambling can make a lot of money in a short time and with little capital, so many people who want to get rich instantly from gambling
Yes Gambling can make lot of money at short period of time but whenever you see it for long period of time always it results to get loss. But it is very easy to use and do not require any hard work so most of the people got addicted towards it. When you got addicted then it is always tough to get out of it.
The number I always hear is 90% return on average over time. So if you gamble 10k per year, you're likely to end up with about 9k.
The fact remains that gambling is a form of Entertainment, not of income (for most people). Obviously you can be a professional gambler if you're so inclined but the ratio of amateurs to professionals is probably pretty staggering.
When it's not fun anymore it's time to stop!