This isn't a rule. Threads are usually locked if they're reported and a mod deems them as run their course.
It seems that some members think that things on this forum take care of themselves, that is, that the moderators patrol the forum and delete posts, move/lock topics and ban bad users. If today we have less than 2000 reports per month, and a few years ago we had record months with almost 50 000 reports, then it is a little clearer why it seems to some that the moderators are not doing their job.
I think most mods will handle something if they come across it but staff only get paid for reports (with some other unknown factors involved) so if there's less reports coming in then spam is more likely to stay unless it gets reported. I barely browse the forum at all these days and tend to just stick to reports and the threads I frequent in my watch-list and I suppose that's probably true for many other mods. I think there should be some benefit for users reporting like badges that come with some special privileges etc as it's a thankless task and it's very rare we add new mods based on their reporting so that's another lack of incentive. Maybe we could even give some small BTC prizes to some of the most active reporters etc.