Kind of by definition we will not see the people who do not submit frivolous PRs that waste the time of other people. So keep in mind that there's likely a huge amount of survivor bias involved.

Just like with email spam I would expect that a big part of the issue is that it only takes a minority of shameless people to create a ton of contribution spam. Unlike email spam these people actually want their contributions to be tied to their personal reputation. Which in theory means that it should be easier to identify and isolate them.