These online gambling platforms have no shortage of money, they were paying out 7 figure amounts to content creators for one stream in some cases. It's possible they can run the whole site through their marketing budget.
Yep, this is my theory as well. The viewer isn't being advertised to by third parties, they're being advertised to by the owner of the platform. If they can get people hooked on gambling, they'll make just as much money as advertisement.