My opinion here is that many people have multi accounts. But they are not caught.
For example: You open a separate btc / eth address for each account. You send your winnings to the stock market and sell.
Let them open multiple accounts and cheat if they want. But, one time they will slip up, they always do, and that's when they will get caught. The fact is you can never make every member here to do the right thing. There will always be abusers/cheats whether you want Admins to make strict rules on multiple accounts or not.
In order to try to fight multiple accounts, it would mean introducing the much loathed document verification which would be the one of the worst thing to be done on a forum like this one.
Take a look at the casinos that have the right to implement KYC at any time, most do not allow multiple accounts but people still abuse the platforms just to get that sign up bonus. Where there is money involved, abusers will be there no matter what restrictions they put in place.