Because they are uneducated. The next time someone tells you gambling is wrong, ask them if insurance companies are immoral. Insurance companies are essentially bookies that will accept a wager on how long you will live, or how long you can go without wrecking your car or sustaining injury. They make educated guesses, set up the ‘line’ and make a profit because the math breaks in their favor. But they also take on a lot of risk. A rash of bad luck almost wiped out Lloyds of London a couple of years ago.
Anyone who decides to go to college instead of work is gambling - they are betting their tuition and years of their lives that their education will result in better jobs and lives. This is by no means a certainty - if the economy should change, your job specialty loses demand, you become ill, etc., you may wind up with no job and a huge student loan debt.
Anyone who invests extra money for retirement is gambling, because they may die before they ever get to use the money.
In short, any time you make a decision with an element of risk, you are gambling. If you leave your steady job to own your own company, you are taking a huge gamble. Yet the typical ‘anti-gambling’ foes would see all of this things as virtues and not immoral acts. Because they don’t know any better.
Then there are day traders and other market amateurs who are gambling every day.
If we arbitrarily limit the scope of the discussion to casino gambling, it simply becomes a matter of risk/reward, or the cost of entertainment. I can go to a casino, and knowing the math as I do can figure out exactly what my hourly cost will be to play blackjack, and what the outer limits are for how much I could win or lose if I get very lucky or very unlucky. If, knowing that, I still choose to play, then I am simply buying entertainment with my disposable income. You’d have a hard time showing me how it’s immoral to blow $5,000 in a weekend in Vegas, when it’s not immoral to blow $5,000 on a weekend cruise along the Riviera. Different people like different things.
The big problem with gambling, other than addiction, is lack of education. People lose much more than they can afford because they don’t understand the risks they are taking, and have no grasp of the mathematics behind games of chance. But then, how many people who didn’t know any better lost huge scads of money when the dot-com bubble burst? They were gambling too. Most day traders will lose money. Many of them will lose everything they own. Are they immoral?
The answer is that you are behaving unethically when your actions have an undue negative impact on those to whom you are responsible. If you are gambling with your kid’s education money and the rent, you are acting unethically. But the same would be true if you blew the rent money and the kid’s education fund on that Riviera trip. The ethical problem comes from the abuse of a form of entertainment, and not from the intrinsic nature of the entertainment itself.
BTW, I used to be a professional gambler, so I know this subject quite well.