I would guess it's because of regulatory compliance. You really don't want to release slots with a major payout but, and if you do, you want to be able to throw the blame at Unity.
The real problem is that the gambling business model is inherently incompatible with revenue sharing, because unlike a video game, you're paying money back to your customers every time they get a small win.
Let's say a gambler is wasting away $500 and getting back 95% every day and he plays until he runs out of money. Then unity would be getting $400 and the casino would be getting $100.