Because sex sells, and it's the easiest way to grow on social media. Have you ever seen the disparity in likes between a female artist's post of just their art vs a post of them next to their art? Artists who get 1k-2k likes on posts of their artwork can get 10k+ when they pose with it. Drawing girls gives about the same results.
I follow a woman who started posting on Instagram last October. All she posts are busts of girls in a stylized, flat colored, cartoony style. They're not bad don't get me wrong, but very much suffer from same face syndrome. Despite that, she's now sitting at around 120k followers, less than a year after starting.
Drawing girls/portraits also allows people to participate in most of the challenges that go around from time to time, like #drawthisinyourstyle, which really help growing your fan base.
