Because, this way we would only have games which can be arrange for 2x win and do that in multiple different places which really doesn't change much if you ask me. Obviously there are people who still like it, otherwise other casinos wouldn't have it, but I feel like more players could come from bigger changes.
For example, Stake sportsbook is easily one of the worst sportsbooks I have dealt with in recent times thanks to their below average customer service and abysmal performance on both mobile and desktop.
This is why these casino sites usually add more in-house games since they are easier to implement with minimal issues.
Don't know if the problem occur since they newly offer this sportsbook option or not but since you encounter this problem with them its better to raised it up on their trade so that they can improve their services.
And I see the sportsbooks is on hype right now since many leagues are opening back again and for sure many bettors are waiting for this opening especially when NBA finals is on the road.