PROJECT - team members, users and groups

It is not clear why we have three different user/group entities attached to projects/tasks.

Would it not be better to allow users and groups to be added to the Project Team and allow the following Project roles to be assigned to users in the project team Project Manager, Project Sponsor,  Project Team Member (default).

Tasks ownership should only be able to be assigned to a user in the Project Team.

sub-project team membership and roles should only be able to be assigned to members of the parent project team.