The money is not the only factor, it's about controlling the public's actions. Money is only useful if it can get you what you want. You can't outright pay somebody to change their mind. You need to use both rewards and punishments to do that, as well as control their sources of information.

If you create a slave class you end up with not only endless money, but also labour and other material desires. It's one reason that people have been heavily pushing for automation as well.