Neuroscientist Jonthan Touboul actually did a study using real math and and observation and everything to figure out why hipsters look alike.
More specifically, Touboul studied the patterns of behavior of any "anticonformist" and determined why they will probably end up looking like each other. Hipsters, although it might pain them to learn it, are a newer edition of the anticonformist trend among people and simply represent the most recent iteration of a recurring human tendency. In other words, they are all people who want to look different, but they want it at the same time.
But the reason all hipsters (or grunge rockers, or punk rockers, or hippies, or whatever) all look alike could be that they all want to be different at the same time that they want to have the security of a group. There is a powerful magic in the drive to be the one to hear the Ultimate Hipster Nirvana (State of Being, Not Musical Group) Phrase, "Hey, who are you listening to? I've never heard that band before."
Because hearing it means that you are unique, and other people like your choices. The ability of the human brain to want two contradictory things at the same time should not be underestimated.