I’m a small breasted woman who does not wear a bra. The reasons of practicality and comfort are described below for bigger chested people (I didn’t see mentioned that if they are really big and you try to run without serious, sometimes two sports bras on they can fly up and hit you in the face). I do sometimes accidentally hit my nipple on something, which I don’t like.
But in my experience, people get upset at seeing boob shaped boobs. Strangers tell me I should be wearing a bra. They want bra shaped boobs. Nipples are scary.
I haven’t worn one since middle school when it was new and fun and made me feel “grown up” (but uncomfortable). But some years back I got shingles and apparently permanent nerve damage on the nerve group that would be right under the bra band on my back and anything irritating it makes me feel like I’m on fire so it’s just not going to happen. Certainly not in the name of sparing the general public the scandal of seeing boob shaped boobs in a loose T shirt. At the very most I will use silicone tape to hide my nipples in very formal situations.