It's not free. - It takes time and effort and work, sometime a small amount for high return, sometimes not, but it's still work. Many people don't have the time nor desire to add more tasks to their already busy lives.

While the Cap One card is a nice example of how to get a great return for minimal effort, it only worked if you already had a healthy balance in another account. Not everyone has the knowledge to get 100k easily (like the BA promo) and when faced with the task like spending $10k to get it via Citi AA, it becomes too much. They see "spend $10k" and back away

When you tell them of other means to "spend" the money, most get nervous and think it's not legal or there's some sort of catch. People like easy, not complicated, and aside from the quick bonus miles, much of it does take an amount of time and effort

Even something as simple as a coin order can require someone be available to sign for the package, having the time to ferry them to a bank, having a bank that'll take the coins, etc

On top of that, most people have been convinced that it's far too much work to get an award flight. They've been told over and over again that they'll face constant blackout dates or high fees or some other hassle