Okta and Auth0 have mostly been serving the opposite ends of enterprise login, with Okta starting to take on Auth0 (and not so much the other way around).
Okta mostly handles being the "enterprise database of users + gateway to your enterprise apps", i.e. it's an identity provider (IdP). ActiveDirectory and PingIdentity are competitors here.
Auth0 mostly handles the other end of the line: they help companies be "a thing you can log into using an IdP", i.e. they help companies be a service provider (SP). Custom SAML/OIDC implementations are the biggest competitors here. Okta has increasingly been competing with Auth0 in the space of SaaS SP solutions.
So it's in the latter half -- service providers -- where the two compete. You may have been looking in the wrong place. Plus, not all companies are service providers, and many companies that are service providers roll their own implementation of the relevant protocols.