I will not say they are fake, they are real streamer who are affiliated or sponsored by casino.
Coming up to the balance they use when they are streaming, high possibility that it is sponsored money that cant be withdrawn.
Just a guess ofc because we never know about how it works when streamers are sponsored by casino or being paid by the casino as part of marketing strategies of the casino.
Why they are so popular? Because they are popular people already in the streaming world before starting to be paid by casinos.
Other reason is that most casino streamers provide givewaway everytime they are doing stream, that's why many people watch the stream.