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.
It should be pretty obvious why casinos don't want people to have more than one account - there is no good reason that you should ever need one. The historical reason that people have tried to create multiple accounts is to take advantage of special offers or promotions that are specifically intended to be one per person. The casinos have worked out their budget for these incentives carefully and they want to draw in as many people as possible with their offers, if one person is taking the amount intended to create many other customers then they lose a lot of money. The odds the house calculate are also based on a single player, you would be increasing your win rate on certain games if they allowed this.