Even people who insist they don't care what others think tend to dress, talk, and act remarkably like the specific group of people who also insist they don't care. Not caring is its own kind of fitting in.
Adults like to think they've outgrown peer pressure. The instinct never actually leaves. It just trades hallway dynamics for boardrooms, group chats, and what everyone else seems to be doing.
Corrections never spread as far as the original rumor. That's not a flaw in human nature. It's a survival bias that made sense for most of history and now runs wild at internet speed.