Perhaps you enabled the filter feature for ads, so you've never seen gambling ads on Facebook. Or maybe the group or people you know have nothing to do with gambling, which keeps you from seeing gambling ads. But it looks like Facebook has its own filter for gambling, and Facebook is very selective with the ads that appear on its site.
Because the reason I'm not involved in interactions about gambling makes my facebook account not related to any gambling, I may not know how the filter works but I don't use Facebook other than to communicate with friends and only take a moment to get information about the latest news about world conditions.