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.
In countries where slots are regulated, if you want to get the homologation you have to respect certain limits. In my country I recently read that it is just over 40% the money the machine has to pay back to meet the standards (ensuring the owners profitability). The problem is that even when there is regulation, the distribution is uneven, and most of that money is won by just a few very lucky winners.