It's pretty simple, and really not caused by people attempting to appear unique. There are two main factors: 1. All humans has a need to be accepted and approved of by their peers, 2. Humans also have an essentially identical neural architecture and a very similar set of likes and preferences to their peers. This results in groups of people with similar habits, clothes, and so on. The vast majority of humans normally want to fit in and conform, rather than be the odd one out in a group, see the entry on tribalism in any good anthropology textbook.
Also, anyone who claims not to understand this is either lying, engaging in some form of virtue signalling or has some sort of mental illness.