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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.
You seem to be very confident in your words... but then how did other casinos that have a swap option handle it? Today in many casinos we have to wager x1 deposit, and in some even more. This is how casinos comply with AML rules, exchanging coins from one to another doesn't have anything with AML. Basically, you can't deposit one coin, exchange it for another, and withdraw it... you need to wager.
i agree with @iv4n on this one. AML have nothing to with this. the reason why not all casinos have a swap feature is because it's not as simple to add as you may think.
for small casinos at least, managing liquidity for multiple coins + the house bankroll adds complexity, costs, and dev hours that could be spent on better features to grow the brand. all that makes the small revenue from the swap fees not worth it from a business point of view.
it only makes sense for big brands, as they have larger deposit/withdrawal volumes.