Okta User externalID usage with SCIM

Hello,

In this portion of the Okta documentation for SCIM v2.0, there are two examples of requests and responses for creating a user. The second example shows a scenario where the SCIM server has custom attributes that it wants to apply for each user entity. It demonstrates how the custom attribute is included in the reponse to Okta, which makes sense. What doesn’t make sense is why the externalID is left out of the request and is instead also included in the response to Okta. The documentation even points this out specifically:

In this example, the externalID and userType attributes weren’t included in the original POST method request, but are generated and returned in the SCIM server response.

Why would custom attributes mean that the externalID should not be included in the request and why would the SCIM server be responsible for creating an externalID? I would have thought that all Okta users already have an externalID to begin with and can be used in every request.

Thank you for your time!