Might be to avoid getting doxed by someone who gets pissed at them in a month or two for an innocuous post. Facial recognition software will ID a face pretty quick online.
Similarly, some probably don't want their business partners to know they hang out on a sportsbetting site. Might make them worry about where the money's going.....
Some may not want their wives to know. Like a said posting a pic opens the door for being "found" online.
Some may be fugitives from the law. Gov't software can search for pics easily online.
Others may be famous and hiding behind an avatar to avoid publicity.
Just saying there are some valid reasons for not posting pics.