As mentioned, casinos often cite security as the reason photography is prohibited, but the real reason they don't like photography is related to customer privacy. Casinos know people are often in casinos that shouldn't be, and are often with people they shouldn't be with (like mistresses or even prostitutes).

It's free publicity for the casino and it encourages other players to keep gambling in the hopes of being featured in a news article or on social media. All with your consent of course with a picture for you holding a big check.