Yeah, it's true social media like Facebook or youtube have strict policies against the gambling related promotions that could be one of the reason why we don't see them but I have seen stake advertised by many creators like shoutout for them for being the sponsor of their video so they exists in social media too but in different way.

Other reason, we crypto commuinity seek open source softwares for our needs which are created by the community and doesn't generate profits so we can't expect them to advertise their products by promoting money from their own pocket which is why we see some of those services has voluntary signature campaigns where users will not be paid for promoting them.