casino sites have another reason for not permit multiple account, welcome bonus and refer bonus could be first reason, because peoples cheating easily to get such bonuses,
and kyc is might be another reason, if casino ask kyc documents, when how you will submit for multiple account, exchanges allows multi account, because they have no offers,
but you can't use without kyc, casino don't allow multi account because money laundering case.
This can be understood very easily with a simple example, Imagine a person opens an account on a casino site,He gets the welcome bonus and even makes some profit,Then he thinks if he opens another account he can get the bonus again,In this way he starts opening multiple accounts just to take bonuses,,For the casino this is clear cheating and causes direct loss,That is why they stop multiple accounts from the beginning,
Another issue is KYC,A person has only one real identity, So how will someone submit documents for multiple accounts, This also increases the risk of money laundering,That is why casinos do not allow multiple accounts to protect themselves and keep things fair,,