Why can't I use an Atlassian Team in an Advanced Plan rather than having to create a Shared Team?

[Christian.L.Sargent]

I've come to learn that the Atlassian (or JIRA) Team one creates is completely independent of the Shared/Private Team one creates in an Advanced Plan.  Is this true and if so why?  It would be great, as an option, to be able to use the same team definitions in both places without having to create both.