It is possible that the streamer is sponsored and he can't withdraw the amount because of the agreement with the casino. It is possible that only a percentage of those winnings goes to the streamer and it needs to be sent by the person of the casino in-charge of the agreement. It is also possible that they are bragging about their balance and pretend that they unconsciously showed their bankroll balance.
Same here, I always withdraw my funds out of the casino whenever I am done with my gambling session. I do not intend to leave any of my coins and use the casino as a stash.