The vast majority of the population on Earth these days makes their money by trading their time for some employment pay.
That means that money is scarce as it is a fixed amount every month.
But rich people make their money from investments, which are generating more money all the time, independent if the rich person works or not.
That's a massive difference, and it makes sense that employed people would have fear of losing their money, because there would be no ways to pay for accommodation, or food, etc.
That is a really good point. Unless this employed person always has some extra money to spare, then it can be difficult for people with fixed income to gamble without being scared or worried. This is why you should just always spend on whatever your salary can bring you. This is not just for gambling but in life in general. Knowing where to spend your money will benefit you even in the long run.