I think most casino websites are already optimized for usage, both on mobile and desktop so there'd be no need for apps? And they target an audience that uses both after all, or heck any sort of similar device so limiting it to apps may kill some of their userbase, not to mention that I reckon it'd be much more expensive to develop an app compared to a website (which btw, has already a LOT of variations so it's much easier to get an available general source code and move from there).
Upgrades
Websites are much simpler to improve than mobile applications. Even the proprietors of casinos can make adjustments in a matter of days, and those modifications will immediately become available to all of their customers. On the other hand, updating apps is a more involved process. It will take some time, and users will additionally need to download the upgrade on their own before the new function will be accessible on their app. You will see that websites are regularly updated, but app users frequently need to write to the developers to notify them about the platform’s faults. It’s said that websites are way easier to modify than applications.
I'd highlight this really. Also imagine if they made an app for the different devices out there (iOs, Android, desktop), pushing updates would mean 3 (or more) separate renditions and adjustments. Pretty easy to see which is more easy and cost efficient than the other.