I think the main reason why some services would advertise in the forum is because it can't advertise elsewhere.
I do not agree. The only reason any seller advertises his site or service on a platform for long time if it is converting well for him. I am very much sure that these advertisers are getting good returns with the investment they are making with the signature campaigns. Then only, they are able to continue the signature campaigns for long time. The forum converts well than other platforms because traffic is very targeted. Everyone who is member here has almost used crypto on daily basis. That is an ideal customer for these type of advertisers.