Well, many reasons, to be honest. Yes, you can easily set up some local LDAP/AD/Whatever and setup some OIDC/SAML connectors.

SaaS like Okta, OneLogin, even Google Directory offer many things that you don't have to think about:

- If your organization doesn't use SCIM and you have headcount that requires HR, then your IT department is wasting time. You also probably will fail audit.

- How do you limit which users allowed to use a service that uses OIDC or SAML?

- Where is audit log stored?

- What's your SLA?

- Does it integrate with HR software?

- Does it restrict access based on location? Is there some logic to prevent a user from city X suddenly login from a city 3000 miles away?

- Do you have contractors? How do they access resources they need?

- Service accounts?