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 am not sure which casinos you are playing, but from what i know, most of the casino would give you huge first deposit money and other offers for new users and some offer faucets and if they allow multiple accounts they will be abused and many casinos have that bad experience and so is the reason they are specific on catching users who are trying to cheat them.