The problem is visibility. When a Red posts about a bug, confirming that they know about it, who sees that? Not many people. Those of us who stalk the Red Tracker, and anyone who hops into that thread. That's about it. People looking to see if Riot knows about a bug can't easily find that information, and posting in threads on the Boards doesn't solve that problem.
A good solution to this would be a "Known Bugs" thread, where Riot updates and records all known issues, so that people can look there first before posting.
Hilariously enough, such a thing already exists. Riot has a "Mega Bug Thread" on the Report a Bug forum, where they're asking for reports on bugs since 4.18. They don't seem to be updating the main post, but it has almost everything mentioned in it. However, since it's stickied at the top, and no-one reads stickies, reposts continue.