I would rather phrase it as: "casinos and slots machines are intended for the house to be profitable". And in order for that to happen most gamblers need to lose their money in the long term.
I read it somewhere that the percentage of people that are making money from gambling is 3%, but I also saw that it may be up to 5%. That means 95 to 97% of people are losing money while gambling. If that is true, I think what OP said is still true also but you are just more accurate. I said OP is right because some people just want to earn money while gambling and that was the beginning of their losses which might even lead to gambling addiction. But having it in mind that you will most likely lose, you will have it it mind to use just small amount of money to gamble.
Either 3 or 5%, still, the percentage is low considering the number of gamblers worldwide. It means, casinos are indeed earning good amount of money. Hence, this type of business is really a lucrative one. Whether you play casino or slots, they are relying both in luck. This is why, no one can guarantee such winnings on this type of gambling game.