Some online casinos make a killing...but usually only the ones that are established and have credibility.
Too many are always under investigation and are rife with complaints about game fixing, manipulation, and not making payouts as advertised.
Then there is the fact, that if you are a US citizen, and trying to operate an online casino - which is ILLEGAL in the US - there is always the ongoing pressure by the US to find a way to tax if they cannot shutdown an offshore casino - since that is where you would have to host it.
Yes, there is an allure to owning one...but there are some serious risks to balance versus potential profits.
Tons of research legwork is the name of THIS game.
My 2 cents