...Is it fiat payment methods? Is it that casinos holding users money? — these two are top of my head, you tell me more.
The primary reason is because they are in position to hold users funds, not just funds but huge funds in their platform. Every platform that holds users money inside must be regulated. Other reasons are secondary whereas important too.
Also, age (must be 18), criminal activities, customers security, and taxation are part of the secondary reasons why casinos prioritize regulation.