You got a good point. The casinos should think about it. If I am not wrong, I have seen some casinos like that. One of them is Metawin, which provides rakeback or cashback no matter your level. Probably, casinos think it is going to be expensive. But they should understand that since the player lost their deposit and their PNL is negative, giving them something back will keep themself on the platform. I used to see players screams on the chat saying a casino is scam because they did not provide any kind of cashback or lossback when they lose their deposit. The casino can avoid these situation providing cashback for every level.
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.
Having different levels for cash back, rake back are helpful for their users, because not all users are the same, have too big gambling capital as well as not too actively with gambling. If no matter big or small, very active or somewhat active, users all have something back from the company, they would feel very happy and can become loyal users of the company in long term.