There are devoted Christians and Muslims that do gamble despite how much their religion prohibits it, you see them applying core discipline to it, and their lives tend to be more pleasing than even those that claim not to gamble because their religion frown at it. Same people that don't gamble still do take alcohol for the stomach sake.
I wouldn't limit it to the only two faith and beliefs. Not just Muslims and Christians that does it secretly. It's very likely that in some other known faith, despite that their teachings is not to gamble to avoid being greedy. There can be other root cause of it when it's outside gambling and yet, the result is the same and they still become greedy. But focusing with gambling, I guess it has become a normal thing when you hear some stories about a faithful or religious guy then have been seen some casino apps on their phone or into their devices.
Certainly not limited just to Christians and Muslims alone as other religions and it's faithfuls aren't exonerated, I only used those two as reference point.
Aside gambling or alcohol, these faithfuls
do default on some other teachings and commandments in their religion that are so immoral but as a result of their hatred for gamble they make it appear as though gambling is the worst of them all, but that's not true. Nowadays, we have much of hypocrite in many religions and it socks whenever my path cross with such people.