Why do most gambling sites are against this style of deposit and withdrawal with different coins?
Maybe that has something with AML policy, maybe not... but I guess that would be a hassle for casinos. It would be complicated for the casino to keep track of all the price changes for the different coins. Maybe if every deposit was immediately converted into a stable currency, and then converted into the desired currency when we ask for withdrawal. From my experience, casinos that convert every deposit into stable currency also allow withdrawals in the coin we deposited, some of them only to the address from which the payment was made.
First and most important, it's because of AML. They don't want to be unwanted engaged with money laundering activities from users.
Next and maybe not last, like you said, they don't want to be in hassle with price volatility of different coins, and even Oracle can have bugs, failures so it is a risk factor for their casino treasury or at least their hot wallets that can be drained out by hackers if Oracle doesn't work properly.