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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.
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Very few casinos have swap or conversion features.
I know, I have experience with most of them... The point is that they exist and that it is possible.