Why would they do that? So annoying.

2 theories:

(1) They couldn't imagine anyone ever closing their gloriously developed MS Teams.

(2) Since everyone knows MS Teams and sitting in meetings all day is the one most important thing to get stuff done, they went ahead and made MS Teams a "priority". F using anything else! Maybe if it doesn't release the audio input, it will be 50ms faster next call! That ought to be enough for you!