Afaik they already thought of that solution because YouTube videos support tagging/categories but it's the video uploader who sets those tags. The problem is t

Afaik they already thought of that solution because YouTube videos support tagging/categories but it's the video uploader who sets those tags. The problem is that once you introduce such a system, and leave all the control over the tags to the uploader, people start gaming it for better search rankings/more views. So you are back to the same old problem of "who checks if the content actually matches the tags".