The main purpose on here is to prevent casino players cheating on their end, that's why the system closely monitored this kind of activities.
I still do not know the reason gambling sites do not let people have subaccounts. Maybe the casino can also use it to encourage people to know that they can have many accounts but in a way that the casino will know the accounts is owned by a single person. I do not think this should be an issue for gambling sites to do.
I understand that people use more accounts to cheat but gambling sites should allow gamblers to have more subaccounts which the users will create through their main account.