Outlook is probably the most widely used, and again, if outlook can be considered part of the office suite, which it has been for decades, why not another communication software?
My broader point being these groupings are all pretty arbitrary.
Microsoft has always sold a cheaper "office suite" without outlook. What businesses have historically bundled and not bundled is irrelevant to what is best for society going forward.
Surely there were many features that many software businesses add that weren't there before. What if Microsoft relabeled Teams to be part of Outlook? Like they made Calendars part of Outlook. It all feels like starting at the result and working backwards towards a justification.
Being around for a long time is a quality of its own. It strongly implies familiarity and good enough fit for everybody. (Traditions compete in a sense, some do not survive)