Use an OAuth Provider

Authentication is hard to get right - see require unleaked passwords for just a few of the challenges around passwords, and the challenges of verifying who is a human, or preventing multiple account sign ups are challenges that even the largest organizations struggle with.

Platforms are better off (when they're able) outsourcing identity management to an OAuth provider. Centralized providers are better able to filter out robots, to prevent users from creating many accounts, and ensuring the handle the details of user authentication in a way that is cryptographically secure.

While this comes at some small privacy risk, and hardens your users' reliance on the OAuth service itself, these harms are small relative to the significant improvements in security and human-representation that platforms can gain through the use of a 3rd party provider.