Fab not taking action solely based on third-party reports is likely due to legal limitations and platform policies. Under the U.S. Digital Millennium Copyright Act (DMCA), online platforms are not required to proactively monitor copyright infringements, and they are only legally obligated to act when the original copyright holder files a complaint.
Additionally, many platforms avoid acting on third-party reports to prevent false claims or abuse of the system. Fab is likely navigating these legal constraints and internal policies carefully, which may give the impression that copyright issues are being overlooked. However, this does not necessarily mean they are unwilling to address the problem, but rather that they require formal requests from copyright owners to take action.
Of course, this is all speculation based on how similar platforms operate. While Fab still has room for improvement in handling copyright issues, I believe that, over time, they will refine their processes and become more responsive. In the meantime, the most effective solution is for the original copyright holder to submit a DMCA takedown request directly. Those who are aware of the infringement can help by informing the original creator so they can take the necessary steps to request removal through Fab. 
Happy nice day! 