Why do "non-conformists" all look alike?

Because without a clear signal of non-conformity, there is no clear identity status as a creative, independent-thinking non-conformist.  There are many ways to send a signal.  Think of them like channels.  The free thinker could express truly original ideas verbally, in writing or another medium, which they often do.  But those channels are narrow, take effort and other people may not be impressed by the imagination displayed.  If a person wants to be thought of as a divergent thinker, the easiest way to send the widest signal, with the least risk of rejection, is personal appearance.  For the visual signal to have effect it must be understood by the recipient, creating, through trial and error, a convergence of “non-conformist” visual cues.  In other words, a cliché or stereotype.  The truly interesting independent would not care what others think of him and thus, visually, may be beyond quickly referenced categorization.