Because it is very simple to do, we just need to deposit into their bankroll.
For some people the return of gambling site investment is not good enough compare to what they get from gambling therefore most people will still just use the money that they deposit in the site to gamble. Some people actually win in gamble therefore they tend to choose to gamble instead of waiting for profit from their investment
It is better to good to be true, returns or profit from investing into bankroll casino's are guaranteed but don't hope the return will be higher enough.
That's how legit investments goes on, it is just like the same thing with banking investments, you invest your money to them and you will be given small portion of their profit for allowing them to use your capital.