Not sure why this would appear to be a rare occurrence to you. We typically delete advertising spam as soon as we see it or a report gets filed.
One reason is my habit of deleting advertising spam without leaving a notification that the ad has been deleted. I tend to do a fair amount of these because I'm often the only moderator on Quatloos for blocks of three or four hours. This is related to two things;
1 - Time zones - I'm on the west coast of Canada, most mods seem to be in mid to eastern America. This puts me three hours behind most mods so while they are tucked away in bed I'm still up and about.
2 - I stay up really, really late. Bedtime usually between two and three AM. So I get the night shift and go to bed when many mods just getting up. I delete most ad posts after midnight Vancouver time.