This has been suggested before now and I remember a comment then drawing attention to how this can become a loophole which can be exploited by spammers and scammers:

A user can create a post and use an alt account to report it for a very vague reason which is most likely to be ignored by the moderator (except they chose to personally look for possibly issues with the post). After they've reported the post, the option is removed and other users can no longer report the post, so it likely remains up. Also other users may think it has been reported already by a well meaning member and not look deeper.