Earlier quoted context omitted.
This is a bad answer. It's actually a non-answer. You can apply this to a lot of questions. Why does X ... Y (because X can get away with it). There are real resources behind Microsoft Teams and a lot of people want it to be good. They may not get much of a signal about how well their product is doing compared to competitors because lock-in effects, but I can guarantee you the product managers and executives at Micro…
Following your reasoning I'd say your answer is a non-answer as well because > executives at Microsoft want the product to work well. IMO, everyone wants the product to work well (except probably competitors). So why doesn't it? I'm aware this is not an answer either :-)
Because microsoft saw competitors that look scary and so decided they needed an all new project released in a hurry. If they have been willing to spend a few years before release they could have a good product, but instead they rushed things to production.
Lync (later skype for business) worked pretty good, but lacks a lot of features (others have mentioned various chat things before then). If they had invested in that all along they probably could have made a stable teams, but the temptation is to call something done and milk profits out of it until you are behind someone more innovated.