Because there is some form of a face to a name here. For some who regularily post pictures of themselves, it's an actual face. For others who don't, their username holds some form of signifigance to others and themselves. Which therefore means that there is still some form of a reputation on here and if life has taught me anything it's that people are very, very fussy about reputations! Plus, real world pictures can lead to serious troubles in the real world. Just like dumb posts on Facebook have gotten people fired, or found out by police officers, and so on.