Facebook’s servers are designed to serve images based on strict authentication tokens. If a user sets their profile picture to "Private," the platform ensures that the high-resolution image URL is inaccessible to anyone not on the approved friends list. A third-party website does not have the authority to override these permissions. If it were that easy to bypass privacy settings, the platform itself would be fundamentally broken.
They often require you to log in with your own Facebook credentials, which allows them to steal your account . fb private profile picture viewer online