Easy, tokens are supposed to be for perk and bonus hunters and those who trade short term
Bitcoin and Ether for those who also wish to hold long term, in the end at least with Bitcoin one can cash out in more exchanges, deposit onto most of casinos regardless of the price, that is the charm of high liquidity.
Yeah casino tokens are limited in number of ways, but Bitcoin,, on the other hand has a universal acceptance and bigger liquidity that make it burn into any transaction.
100% of casinos accept Bitcoin deposits even though they may have their own tokens, but because of the dominant feature of Bitcoin, it is always number one on the payment method of most casinos.
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.
Also, there is the fact Stake or other reputable casino which is properly registered could face some backlash from regulators by minting a token in that way. In jurisdictions like the United States it could be even considered to be an unregistered security.
That being said, a casino as successful as Stake does not have any necessity to launch something like that in order to continue leading the way in this industry.