Yeah, I'm aware of "churning". AFAIK its only possible to beat the system by taking advantage of many temporary offers in series and distorting your spending to get the 'free' value offered by their featured partners. i.e. you can only beat the system to the extent that you value airline miles and gift cards as their cash equivalents.
I'm not even talking about churning, though that can be a thing too.
I need to use something for purchases, and each method of payment has pros and cons. Even if using a credit card had a visible 2% additional cost, I'd use it on certain transactions. If I'm dealing with a vendor I might not trust to do a refund, a cc lets me do a chargeback. If I'm concerned about holding large amounts of cash, a cc lets me limit my risk exposure. If I don't have the cash currently, a cc give me an interest-free advance until the statement due date (between 20-40 days in the future).
We can certainly argue about whether those things are worth 1% or 3% or .02%, but they're worth something to consumers.