As title says,
Since casinos are not giving any kind of free money and they have a house edge that guarantee the win for them in a long run no matter where do you bet or from which account, i.e. They aren’t either losers or winners. It’s usually used to prevent spamming by bots. The idea is that why would a bot use dual account while they can change seeds and do everything from one account?!
I’M afraid there is a something we don’t know about this

If you ever visit scam accusations section, you should have seen atleast one problem from this kind from a very trusted known casinos which looks weird for me.
Ban evasion for multiple account is necessary. Because they provide several types of bonus who are active on bets as well as they provide bonus for the active users on the chat section (maximum site but not all). So, if you have multiple account and claim bonus for all of your account then that is not fair. Is it fair???
Im seeing this to be a reason most of the time where multi-accounts is prohibited specially when claiming a bonus or would be abusing that promotion because it isnt really fair in the first place.
Some gambling sites doesnt really ban out multi account as long you dont do anything something stupid or shady but they arent blind that you do have lots thats why you shouldnt do anything stupid.
So as a gambler then you should be aware on how it works and what are the common terms and conditions of a certain casino.