Maybe some campaign managers feel pressured in to filling slots therefore allow many things to take place which they normally would not. It depends on the campaign manager and also on the business being promoted. I agree, lowering payment and bonuses will probably bring an even lower quality of post from spammers but there is no easy solution except to self-moderate threads in boards where signature spammers and scammers are running riot.
It's not up to campaign manager all the time I think. Sometimes, project representative also mention some rules and among them, I have always seen they have a high number of post requirements in gambling sector. Personally, I don’t like to force users to post on any section but projects requires a minimum number. Campaign manager can recommend to lower it which I did too but sometimes, you can't follow your own. You have to hear the project owner.
There is nothing wrong with the number of posts requirement; I can write top 20+ top quality gambling posts weekly without issue; even if the managers reduce it to 2 gambling posts per week, a shitposter without game knowledge will still shitpost; a self-moderated thread will help the managers, moderators, and make discussion easier. Even worse is word spinning, copy and pasting from bogus sources in the same thread.
Why is there a "Report to moderator" button when they are not blind?
Report to moderator button is not made for managers

You stated that the managers are not blind to catch spammers on their own, so I gave you the "report to moderator button" as an example to drag you through the mud.