With all the remote work going on, my organization has multiple meetings a week with over 100 people in it. It happens quite frequently that someone doesn't notice their mic is not on mute. If you're in a hurry to get into the meeting and you don't give teams a second to check how many people are in the meeting, it's possible to join without it muting the mic for you. That happened to me this morning and I came looking for this exact thing.
If you had asked me yesterday if it was possible to have your mic muted by default, I would have said, "yes, 100% possible." Really shocked to find that it's not something you can set by default. I rely heavily on that setting in other apps for work.