Because it's important for us to root out sexism where we find it, for the same reasons we pull up any obnoxious weed. The fact it wasn't intentional makes it all the more obnoxious.
By replacing "boy/girl" with "child"? Ageism.
By replacing boy/girl/child with "person"? Humanism.
Yes I can go on.
Obviously idiotic.
Make all sexists sew a yellow male-sign onto their clothing.