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.
The use of multiple accounts when playing at an online casino is prohibited. This is no coincidence.
Very often online casinos provide bonuses to new players. Players registering for the first time on the platform have certain advantages. This is for advertising purposes. A person who has two or more accounts can receive two (or more) times as many different bonuses at an online casino.
In addition, a player with multiple accounts can imitate some of the game's actions. This can give him an edge in some gambling.
Thus, the ban on opening more than one account is justified.