Why do we have the 50 posts to be able to PM if someone can just spam Forum Games and people will barely even notice?
Because spambots will simply PM over and over and over again on the board non-stop. PMs are harder to stop and deal with than lots of posting as we can delete posts -- and once it's deleted then nobody can see it anymore -- but PMs are nearly impossible to delete once they're sent. It is also to encourage people to become more active participants on the site.
Also spambots aren't smart enough to try to post and then PM everyone the board. They just join a site and then send out endless PMs non-stop (and sometimes they will post new messages, but not as often as those aren't nearly as effective as spam).