Screen sharing does not mean sharing the whole screen, but sharing something displayed on the screen. Usually one shares only one window or a Workspace (Linux) or a Space (on Mac), not everything. That's why the arguments that one would see own shared screen (part) on another shared screen (part) are not valid. They are only true in the case really the whole screen is shared.
This is not a usability issue. This is primary the privacy question. There can be popups on the screen from different messengers you use, headers from the emails, some sensitive documents opened. That is why it is a bad practice to share the whole screen. It is like setting password to "123456". Users that care about privacy share particular window or Workspace/Space only.
Why MS Teams does not have such function we should as the provider of this tool, Microsoft.
Are there any UX issues that anyone foresees for such a feature addition?
It depends on the context. In some cases having multiple presenters can distract and a single presenter is desired (e.g. a single person is presenting something to the attendees). In other cases it is desired that attendees share their screens simultaneously (e.g. attendees had to perform some task an should show their results simultaneously). That's why having such a feature like multiple simultaneous presenters is an advantage of messaging tool.
People who prefer MS Teams to the messengers like Zoom and who need simultaneous sharing of some visual artifacts use tools like Miro as a workaround (but it cannot of course replace screen sharing).
Why Microsoft has not implemented it (yet), you should ask Microsoft. I would suppose that one of the reasons might be the tight integration of MS Teams with other MS products: Word, Excel, PowerPoint. When a group of people collaborates on a document, they see immediately what other people are doing, in what place in the document. So they don't need to share their part via screen. Another reason might be development resources or costs. Or may be MS analyzed users' feature requests and this one was not as desired as the others.