Why do microservice architectures typically centralize authentication in a dedicated Authorization Server rather than each service handling its own login?

Centralizing avoids duplicating (and inconsistently maintaining) credential storage, password policies, and MFA/social-login integration across every service. Every microservice becomes a simple 'resource server' that only needs to validate a token (usually via JWKS-based local signature verification), rather than owning the far more complex and risky job of authenticating users directly.