Because there are fakes. Fakes that do a really good job pretending they're real. Like me! j/k or am I?
Actually I would rather ask, how old is that picture?
Thank you for best answer.
But is that you in your pic? :D
You will someday.
It's a way of either leading up to a flattering line or skepticism due to the abundance of profile pictures of people who are not quite the person behind the profile.