The uglies will get judged for it and the hot ones will do it for attention.
Plus it looks aesthetically much nicer if everyone has a picture/gif of a popgirl or TV show as their avi, instead of LQ selfies.
And nobody is on this site to 'be who they are', infact it's quite the opposite, most people come here because of their obsession with certain popstars which isn't something you'd display in everyday life.