The more generous their policies on rake back, cash back are, the more money they will have to spend to users. It's more costly for them, but as a business, if they can satisfy customers, customers will be very more likely stay with them.
Casinos might have different strategies. But rewarding a little extra bonus is not a loss for the business. They will always get a good return if they do this as an investment. To make players loyal, the casinos should always provide something exciting so their players continue to play on their platform. I believe you know why people try different casinos, they look for something exciting and sometimes for bonuses. But if a casino continues to reward their players, they are likely to become loyal to that platform. I don't consider bonuses as expenses, I think it's an investment for the casino. Because they are going to get a return in the future.