A lot of replies here ignoring the obvious - patriarchy, religion, and capitalism.
There are plenty of examples of societies past and present, for the vast majority of the time humans existed probably, where breasts are free to exist as they are. The people those breasts are attached to do all manner of work and activities without hinderance, or chafed nipples. Just like those with flat chests do.
The reason that in (especially our western) modern world we are expected to wear bras has to do with puritanism, shame, control, and profiteering from all of the above.
Comfort is categorically not a priority of the billion dollar bra industry, the trillion dollar advertising industry, nor those who demand we wear them to be “respectable”, lest we “excite” their, or their son’s, urges or “distract” them from their important manly duties (and if we don’t, and they attack us, it’d be our fault for not being “modest” enough and “asking for it”).
When we have all been socialised with these demands and expectations for centuries, they become so deeply ingrained, it’s really hard to separate from our own free will, and yet ask any group of people who wear bras what the best part of the day is, and they’ll tell you it’s taking it off.
And to be clear - this isn’t a judgment of anyone who wears a bra, I wear an underwire once in a blue moon which I hate, but am usually in a sports bra because I feel more comfortable with everything held in place (and also have nosy neighbours I’m not that friendly with), it’s about questioning why I feel more comfortable that way, and how much of that is natural vs manufactured by a society that demands I keep everything held in place, hidden (unless they’re “required” to sell something or entertain the mens), and forever impossibly youthful and perky.
Anyway, I’m stoned too and I tend to ramble, so I think I 'll leave it at that for now lol