Why is it only possible to select shared teams for the team field in Jira?

We cannot use shared teams on our instance (~10k User) , that would be a total chaos, so every PO / Project maintaines their private teams in Portfolio, which would be totally fine.

But these Teams cannot be selected in Jira

Although the user has the rights. Why is this different to e.g. the field "Parent Link"?

The message is

"Plan-specific teams can only be selected through a Portfolio plan"