There are several ways to look at it...
I would not say gambling and slots are designed so all gamblers will lose their money, as there are still gamblers who manage to pull some wins off those machines.
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.
Gambling is a business after all, not a charity otherwise the edge of the house would be negative and in favor of the gamblers, that is not going to happen.