I think your attitude that going out in public is tacitly opting into interactions with strangers is a much more gross way to operate. The assumption that it's easy to just politely decline a conversation (and that not doing so in the form of a conversation itself) seems like an extremely narrow-minded point of view based on your own subjective experience. You're conflating social anxiety with the desire to "assert oneself", when it's closer the opposite; socially anxious people quite often don't want to assert themselves, which is exactly why the "just politely decline" strategy is misses the mark so badly. The fact that wearing the earbuds opts out of that passively rather than actively is the entire reason it's desirable.

In one of my other comments in this thread, I explicitly called out that this desire has nothing to do with like or dislike of the people who I might have social pressure to interact with. Some people find social interaction a net expenditure of energy even with people they like, and having to do that repeatedly throughout the day because I want to go to the doctor or something and society has decided that it's "rude" if I don't engage with literally anyone who happens to want to talk to me when I'm in public is honestly just silly. It's not like I'm keeping the earbuds in and refusing to talk to anyone when checking in at the waiting room; I just don't care to have to have a chat with my Uber driver or strangers on the subway while I'm out, and it's ridiculous to imply that I should just never go in public if I don't feel the way you do.

What is the public if not the space where we interact with others?