It's their joint effort to show that the streamer is making massive profits while in reality most of us lose money.
It's a strategy being used by many companies and works out well. As a user we have to be aware and only gamble within our budget.
There is a thing called "streamer luck"
I used to play a hack&slash game where you hunt for items. A regular user can try to farm a certain item for 1000 hours and get nothing but then you'll see a streamer getting 2 or more in 100 hours. Then people found out that the game devs and certain streamers were in an agreement and cheating. It means the devs raised the drop rates of the streamer accounts.
The same thing may happen with the casinos. You'll never know.
Most highest rollers / winners boards you'll see on online casinos are fake too. It is a marketing trick. Lying works.