Because a lot of idle, hard up people full of neurotic energy come to the site, and it's distracting or encourages...unhealthy behaviors. Look, some people honestly have issues, and even gross chronic masturbators should be able to come to the site without worrying about getting assaulted by sexualized imagery constantly.
Further, while some folk who use the internet may be numbed degenerates, like the sort of low stock that regularly consumes anime/cartoons, for people who haven't abandoned all restraint they don't enjoy coomerism seeping into everything else they do any more than they'd enjoy people pouring high fructose corn syrup in everything they eat.
I fully approve of captain J's stance on coomer pfps.