You can flag them because generic tangents are against the site guidelines. It might help, but when they're at the top of a thread, the upvotes will most likely dominate the flags. That's why moderator intervention is needed.
Btw, a lot of the time these comments are perfectly fine as long as they're halfway down the page. Humans like to talk and we naturally talk about what we already know, and repeat things we've said or felt many times before—it's just the way we are. The problem comes more because people upvote the familiar (because hey! I feel that way too!), and then suddenly the more boring category (regurgitating the familiar) chokes out the more interesting one (new and unpredictable information). In most cases that wasn't the commenter's intention. The solution is for a countervailing mechanism, like downweights, to dampen the effect of this suboptimal one. Unfortunately that still requires moderator intervention, which is a bottleneck.
Btw, a lot of the time these comments are perfectly fine as long as they're halfway down the page. Humans like to talk and we naturally talk about what we already know, and repeat things we've said or felt many times before—it's just the way we are. The problem comes more because people upvote the familiar (because hey! I feel that way too!), and then suddenly the more boring category (regurgitating the familiar) chokes out the more interesting one (new and unpredictable information). In most cases that wasn't the commenter's intention. The solution is for a countervailing mechanism, like downweights, to dampen the effect of this suboptimal one. Unfortunately that still requires moderator intervention, which is a bottleneck.