I don't really know the answer to that, but I'd assume it's because the gambling section is covered by global moderators, which probably have a lot of other reports to deal with. Although, I suspect every report is reviewed, it would be nice to have a dedicated moderator that actively participates, and removes the spam as they come across it.

However, it's a absolute huge undertaking to fully moderate the section. You could probably get it to an acceptable level in a few months, but there's so many replies in those megathreads, and if you don't watch the sports, then sometimes you don't know when those users that are basically just putting vague replies out, are doing exactly that. Sometimes you can, but ultimately to be able to catch the majority of them you need to know a little about the sports.