Why most casino apps aren't on the App Store or Google Play
Apple and Google almost entirely ban real-money gambling apps (casino, sports betting) from their stores, except where the operator holds a specific local license for the country the app is installed in. Since most international casino sites operate under Curaçao, Malta or Kahnawake licenses only, they fail to meet that bar — so the "only" app available is usually a WebView wrapper downloaded straight from the operator's own site, outside the official store.
This is entirely different from private poker club apps like ClubGG, which are technically classified as skill-based games within a closed community and are therefore officially available on both the App Store (iOS) and Google Play (Android).