As others have mentioned, solutions like Cognito and Auth0 are primarily focused on providing authentication (authn) and stop at basic RBAC when it comes to authorization (authz). Many companies quickly outgrow RBAC or need more powerful authz models (fine grained, attribute based, etc.) to serve their business needs.

Warrant focuses on the authz piece, supporting RBAC (like Cognito & Auth0), but more importantly, allowing companies to define & enforce their own authz model.