You are absolutely right, and to add, I personally have not come across any casino that listed a token that belongs to another casino on the platform, I believe it's safe to say that this is due to the heavy competition among online casinos, every one of them is trying their possible best to become the best, and I believe they probably see listing a token belonging to another casino as helping such a casino to climb up, which is something they don't want to do, this is just pure business ethics I believe.
There are also not so many casinos that have their own tokens (I only know 4) and as you mention, it makes no sense at all that another casino would list a competitors casino token. Most of these tokens are also only traded on smaller exchanges so the liquidity is low which could be another factor why not to list these tokens.