Based of my experience.

Multiple-account are most the time from "Sportbook" user. Why? even they're not taking any bonus, they trying to multiple-account the casino because new account is always have limited amount to be betting for sportsbook.

That's why, they multi-account on the site. Sometimes, there has some unwritten rules in casino (Multi-account can be acceptable as long the user not using anymore the old account or trying to abused promotion or referral).