Gambling isn't an investment but if you take part with those bankrolls investment, you are investing on the casino and you're with them.
But if you gamble alone, you're the customer and gambler where they are earning.
The best way to invest in gambling is really by owning the store or company. To start, you will need a lot of month though but eventually, you’ll be able to pay winners from the money others lose. Imagine the company that drake lost $1 million to (I know it isn’t always the case).
It's not the best way because if you don't have that capital, you see how it is going to be, you'll not be able to. But if you have some small amounts and they're spare, you can invest to their bankroll and be with the casino and win through house edge against the gamblers.
While your point may be valid, understand the difference between the best way and the easiest way. Assuming you have all the money needed, the best way to make the money is by opening the casino. But if you don’t have the money to launch a casino, you have to go for the easier way due to your hinderances. You see?