There are a lot of gambling sites which are a scammer and those sites who offer investment plan like a Ponzi is too much risk, we should avoid those types of Ponzi scammers.
There are casino's that are coming out like this but if a legit casino offers bankroll investment, don't think that it's like this. There's more for these casino's to tune in because they are legit operating and most of the investment opportunities they offer are all legit.
Reminder: this is for those casino's that are reputable.
I think nowadays we have more casinos that are legit and most of them are not accepting investments.
Stake and primedice are good casinos and as far as I know, they never once made an investment open for the public.
They are still profitable now and they are willing to take the risk since they are confident that their site will survive and will continue to operate profitably.
Those who are established well doesn't cater or offer investments anymore but before during the early days, most of them does. The two casino's you mentioned are well reputed and has a very reputable owner. They never made any public announcement for investment because they already have enough budget to cover their operation and expenses. I wouldn't guess on how much the budget for those 2 big casino's you mentioned, primedice is well known and has been operating for years too.