IMO the reason why they're not that common on most casinos is because there's a risk that it won't be profitable since their rates are always higher compared to regular exchanges and that amount of money is probably better spent on other things like marketing or promotions.
True,setting up an exchange like system cost more than the ordinary crypto casino but some casino had done it anyway.
The high cost actually hinders the casino in building their own exchange though it will surely be convenient to their users. There are a few sites that have added this feature but there are also casinos that are struggling to build one. If their focus is just to maintain their reputation and to keep their business growing, building up an exchange on their site won't be their priority. If more casinos will try adding an exchange feature on their site, they can keep more players who want to save fees from transacting in other exchanges.